I get abnormally tired around two hours into my trail runs. What can I change to improve my endurance? -Kevin, Santa Cruz, CA
I hear these sorts of complaints all the time. “Running is not usually too hard, but long runs are impossible.” There may be some easy answers. Make sure you’re training consistently, not doing excessively intense runs all the time, and adding some structured workouts to supercharge your fitness. But what happens if you do all those little things mostly right, but long runs still turn out all wrong?
Here’s a secret that could unlock your long-distance potential. “Endurance limits” are sometimes just related to inadequate fueling relative to intensity level.
For example, think about Damian Hall and John Kelly, who both set FKTs on the 268-mile Pennine Way in the UK in successive weeks. In the training blocks for the FKTs, neither of them went beyond 27 miles in a single run. How did they make history over a distance that was nearly ten times longer than their longest training run? They did it by slowing down so that they could replace fuel they were burning, while hydrating adequately too. All day runs require an all day nutrition/effort combo, which is dependent on burn rate and relative intensity.
On a smaller scale, every long run we do is like that Pennine Way adventure. We go farther than usual, but our bodies can handle it with small changes in fueling/intensity strategy.
Let’s start by thinking about glycogen, which we’ll simplify to mean stored calories that are used mostly during moderate or hard activities. Aerobic threshold is the range of intensities when athletes switch from primarily glycogen metabolism to primarily lipid metabolism. The body can burn fat all day, though even very low intensity activities usually involve some glycogen metabolism. When glycogen levels start to move toward empty, the body reduces output to conserve stores even before the dreaded “bonk” strikes. That hollow, fatigued feeling that many athletes describe is related to the fuel needle moving toward empty.
Studies show that athletes can store approximately 1000-2000 calories in glycogen and replenish around 300 calories of glycogen per hour, though it varies based on background and training. A hard effort might burn up to 1000 calories per hour of glycogen in some athletes (though it varies), with less intense activities burning a lower proportion of glycogen relative to fat. Given that replenishment rates are slower than the max burn rates, we can only go so hard for so long before bonking, even with mid-run fueling.
In the question, the timeframe of two hours is significant. That is around the amount of time athletes have at an aerobic threshold effort level before starting to run low on glycogen stores. Glycogen burns rapidly at moderate intensities, but refills at a drip. So it’s key to make sure you’re not burning too hot while also refilling what you can. Many athletes that perceive abnormally low endurance limits are sensing an impending glycogen-depletion bonk. Their bodies are saying “enough of this, time for pizza and a nap.”
Those protective mechanisms are there for a reason, a low-gas light on the dashboard of our 1997 Honda Civics that stops us from getting stranded in the middle of nowhere. Athletes can often ace long runs from 2+ hours to 268 miles by applying three rules.
First, raise your aerobic threshold with smart training. I think most trail running training systems should revolve around improving running economy, making faster running take less energy. If you’ve read my articles before, you get the basics: mostly easy running, fast strides, controlled workouts, applied with long term consistency.
Second, fuel before and during long runs. Start with ample energy reserves, filling glycogen stores the day before (my vote is pizza). Eat a bit before your long runs if possible. But most importantly, fuel your long runs like you would a race. I like athletes to aim for 200-300 calories per hour or even a bit more. Over time, the body will get better and better at processing that fuel. Countless times, I have seen athletes start fueling long runs adequately and have endurance breakthroughs right away. Those training successes feed back into long-term growth, raising aerobic threshold and improving speed over time too.
Third, don’t go too hard. No matter how much you fuel, you can only spend so much time well above aerobic threshold before you start to run headfirst into the wall. As you get confidence overcoming the two-hour barrier, keep it extra easy. Later on, you can start to push it a bit more on climbs, or add steady running and tempo workouts within longer runs. But at first, try to finish long runs feeling like you don’t need to grab a pizza and a nap right away.
There are a few added benefits, too. Adequate glycogen stores will prevent excessive catabolic processes that break down muscle. Fueling will support hormone balance and a healthy metabolic rate, supporting sexual function and overall health too. Finally, it will manage cortisol and stress, leading to better adaptation over time. Make sure you’re hydrating adequately too, which can cause similar endurance-limit feelings, but is an article for another day.
Put it all together, and fueling could be the key to overcoming the two-hour barrier. And long-term, as training and fueling lead to positive feedback cycles at the cellular and systemic levels, avoiding the bonk may lead to limitless fitness.
David Roche and his wife Megan are coaches for SWAP Running, and they host the Some Work, All Play podcast.