Fueling for the Marathon Half-Marathon

No-Fuel/Slow-Fuel Long Runs


You’ve completed a few marathons and now want to finish faster. But, it’s not like you can turn pro and run 100 miles per week. You need an efficient strategy that can fit into your current training schedule yet deliver big performance improvements. No problem. No-Fuel/Slow-Fuel long runs are a proven way to improve your marathon performance and they work for everyday runners like you and me.

Weaning Yourself Off Fast-Acting Sugars

No-Fuel/Slow-Fuel training, also called low glycogen training, results in greater fat burning, more muscle fiber recruitment, a boost to the aerobic system, a lot of mental toughness training and greater storage of muscle glycogen post-run. All of these adaptations are extremely helpful in helping you race faster in the marathon.

But, if you normally fuel on your long runs with fast-acting sugars (sports drinks, gels, etc.) then you need to spend some time weaning yourself off of this type of fueling. For most runners, it just takes 2-4 long runs till they get used to the No-Fuel/Slow-Fueling.

The No-Fuel/Slow-Fuel strategy relies on running with compromised glycogen stores, avoiding spikes in blood glucose and enduring high mental fatigue so you want to avoid traditional fast-acting sugars during No-Fuel/Slow-Fuel long runs, which should be most of your regular ol’ long runs for distance or time.

Note: You might still use fast-acting carbs as part of your race day fueling strategy and practice them in your specialty workouts and tune-up races. For many runners, the winning formula for marathon fueling seems to be slow-acting carbohydrates for at least half if not three-quarters of the marathon then supplementing the slow-acting carbohydrates with fast-acting sugars (and caffeine) over the last few miles. But as with most things in running, you have to experiment to see what works for you.

Why would you make your long runs more difficult by limiting fuel? Think of it like hill training. You purposely choose terrain (the hill) to make running harder so you get certain benefits from hill training. You do the same thing in No-Fuel/Slow-Fuel training. You compromise your fueling to make the body work harder and as a result stimulate greater adaptations—metabolic changes which will help you get through the race with plenty of fuel and thus avoid the bonk.

Easing Off Fast Fuel

To get stated with No-Fuel/Slow-Fuel training, first reduce or eliminate fast-acting sugars in some of your shorter long runs before you begin your marathon plan. Or you can start during the first few long runs of your plan. If you have a two-hour long run scheduled and would normally ingest a gel at 45 and 90 minutes in the run, for example, only take one gel between 60 and 90 minutes into the run or do not take a gel at all and just hydrate with water and electrolytes.

After just a few long runs of weaning yourself off of fast-acting sugars, you should be able to run for up to two to two and a half hours with just water and electrolytes during the run. You’ll have no need for carbohydrates.

That said, some runners don’t tolerate running with low blood glucose levels so using slow-acting carbohydrates (like a serving of UCAN) every hour to an hour and a half in your No-Fuel/Slow-Fuel long runs is a great way to wean yourself off of fast-acting sugars.

As you get fully into your marathon training plan and your long runs become longer than two to two and half hours, begin using slow-acting carbohydrates (as well as water and electrolytes) during your runs. I find that going with no fuel for longer than two and half hours creates too much lasting fatigue for most, but using slow-acting carbohydrates provides similar adaptations yet doesn’t hamper your upcoming training.

Please note that the No-Fuel/Slow-Fuel strategy is only for your long runs where you are just running for distance or duration. The No-Fuel/Slow-Fuel strategy is not to be used for your specialty, marathon-specific long runs, like long runs with a fast finish, goal pace running or runs with surges/pace changes. In those specialty long runs, you’d practice your planned marathon race day fueling strategy.

Next Level No-Fuel/Slow-Fuel Training

If you’re comfortable with No-Fuel/Slow-Fueling for your long runs and want to take it to the next level, then you can begin to eat only a low-carbohydrate breakfast before your long runs or even skip breakfast all together. Naturally, this further reduces your fueling and enhances the low glycogen stimulus.

Some runners even take it a step further and eat a low-carbohydrate dinner the night before to further lower the carbohydrate stores going into the No-Fuel/Slow-Fuel long run the next morning. This can be enhanced by doing an easy run the evening before the long run.

Hardcore low-glycogen runners add a harder, glycogen-depleting run the evening before their long run—either a moderately-long run of 60-90 minutes or a VO2max workout like 8 to 10 times 1 minute at 5k pace with 1-minute recovery jog between. This definitely leads to a strong low-glycogen stimulus, but introduces a lot of potential fatigue so it’s only for runners used to low-carbohydrate eating and training who are never injured.

No matter how aggressive you want to get, the end goal is that you force your body to run with low glycogen stores and your mind to tolerate running while feeling fatigued. Any of the strategies for weaning yourself off of fast-acting sugars can help you get the benefits from No-Fuel/Slow-Fuel training, and will make those runs where you do practice your race-day fueling strategy feel amazing.

5 Stages to Wean Yourself Off Fast-Acting Sugars

Stage Strategy Example
1a Reduce fast-acting sugars Use sports drinks and gels less frequently. Hydrate with water and electrolytes throughout the run.
1b Use slow-acting carbohydrates As quickly as you can, eliminate sports drinks and gels altogether. You can use slow-acting carbs like UCAN if you feel you still need some fueling. Hydrate with water and electrolytes throughout the run.
2a No fuel (preferred) or slow fuel for long runs lasting less than two hours For run of less than two hours, only use water and electrolytes. No carbohydrates. If you struggle with this strategy, then use slow-acting carbohydrates during these runs.
2b Slow fuel for long runs lasting over two hours For long runs over two hours (that are just for distance/time and not pace), use slow-acting carbohydrates. Hydrate with water and electrolytes throughout the run.
3a Low carb Breakfast Before No-Fuel/Slow-Fuel long runs, avoid carbohydrates in your pre-run meal.
3b No breakfast Before No-Fuel/Slow-Fuel long runs, don’t eat breakfast.
4a Low carb pre-run dinner Avoid carbohydrates in your dinner the night before a No-Fuel/Slow-Fuel long run.
4b Afternoon/evening run Do an evening run before the night before a No-Fuel/Slow-Fuel long run.
5 Evening glycogen-depletion workout Make the evening run the night before a No-Fuel/Slow-Fuel long run more glycogen depleting by making it a longer run or VO2max workout. Be careful if you are injury-prone, as running on tired legs the next day can increase injury risk.

Warning: Adjust Your Expectations

No-Fuel/Slow-Fuel training requires some planning and an adjustment to the usual long run expectations.

On the planning side, avoid routes that take you far away from civilization for a No-Fuel/Slow-Fuel long run. After all, you’re going to be running on low fuel so if you run into issues, you don’t want to be stuck in the middle of nowhere. And as with any training run, let others know where you’ll be running (or ideally, take along a training partner). Lastly, take some fuel with you. An “emergency gel” gives you peace of mind, particularly if you are new to this strategy, so tuck a gel in your shorts or pack just in case you need it.

On the expectations side: be prepared for it to suck, particularly if you are used to fueling with fast-acting sugars on all of your long runs. No-Fuel/Slow-Fuel long runs are designed so that you run low on fuel. Many runners even bonk towards the end. You need to be ready for this mentally.

Remember that feeling toward the end of the marathon where it was tough just to keep from giving up? When you want more than anything to curl up in the ditch and take a nap? That’s what you’ll feel toward the end of No-Fuel/Slow-Fuel runs—at least for the first few. But, in addition to the metabolic adaptations, that’s part of the purpose. You are building a lot of mental toughness and it will serve you well in the marathon.

Lastly, the pace on these long runs is often slower than your usual long runs—particularly on the first few, and toward the end of the runs. You need to be okay with this and accept it as part of the training plan, not compare it to your goal marathon pace and become concerned.

Applying the No-Fuel/Slow-Fuel Strategy to Your Marathon Plan

How does No-Fuel/Slow-Fueling look across your training plan? The chart below shows the last 10 weeks before the marathon and the strategy you use to get the most from the No-Fuel/Slow-Fuel concept.

You start with no fueling (just water and electrolytes for hydration) during the shorter long runs early in your plan. Then move to slow-carbohydrate fueling as the long runs get longer and longer. Then finally, you alternate between No-Fuel/Slow-Fuel long runs and long runs where you practice your marathon fueling strategy.

Week Long Run Strategy
1 Long run just for time/distance (no fueling if under 2.5 hours, slow fueling if over 2.5 hours)
2 Long run just for time/distance (no fueling if under 2.5 hours, slow fueling if over 2.5 hours)
3 Long run just for time/distance (no fueling if under 2.5 hours, slow fueling if over 2.5 hours)
4 Fast finish or goal pace long run (practice race fueling)
5 Long run just for time/distance (slow-acting carb fueling)
6 Fast finish or goal pace long run (practice race fueling)
7 Longest long run (slow-acting carb fueling)
8 Fast finish or goal pace long run (practice race fueling)
9 Shorter long run (practice race fueling)
10 Marathon Race

If you are unsure of your marathon race fueling plan, then skip the No-Fuel/Slow-Fuel strategy on a few of your regular long runs and instead, practice your marathon fueling plan. It is critically important that you have a tested and proven marathon fueling plan so skip the No-Fuel/Slow-Fuel strategy till your get your marathon fueling strategy finalized.

The same goes for tune-up races, marathon-specific long runs (fast finish, goal pace, pace change) as well as any goal pace workouts within your marathon plan. For all of those workouts, practice the fueling strategy you plan to use in your marathon not the No-Fuel/Slow-Fuel strategy.


While I’ve talked mostly about limiting fueling during long runs, the opposite is true after the No-Fuel/Slow-Fuel long runs. One of the key benefits of No-Fuel/Slow-Fuel training is that your muscles are stimulated to store more glycogen (stored carbohydrate) after a depleting run like a No-Fuel/Slow-Fuel long run.

You should ingest a liquid with carbohydrates and a little protein within the first few minutes (less than 30 minutes) after a No-Fuel/Slow-Fuel long run. Chocolate milk, smoothies, shakes, UCAN Recovery Mix are all good choices. Basically, any liquid that you enjoy and can get in quickly will take advantage of the increased glycogen storage that is stimulated after a No-Fuel/Slow-Fuel long run.

Final Thoughts

No-Fuel/Slow-Fuel long runs work. They take some time to get used to and to learn how you respond and recover, but they are a great strategy for marathoners looking to beat the bonk, break through “the wall,” fix the fade. They are worth the difficulty in order to have more energy—and speed—over the last few miles—and maybe to get that coveted Boston Qualifier.

As always, let me know if I can be of any help.



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Written By Greg McMillan
Called “one of the best and smartest distance running coaches in America” by Runner’s World’s Amby Burfoot, Greg McMillan is renowned for his ability to combine the science of endurance performance with the art of real-world coaching. While getting his graduate degree in Exercise Science he created the ever-popular McMillan Running Calculator – called “The Best Running Calculator” by Outside Magazine. A National Champion runner himself, Greg coaches runners from beginners to Boston Qualifiers (15,000+ and counting!) to Olympians.

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