However, do you want to do this in every long run? I’d say certainly not. You definitely don’t want to do it in every training run. Never mind the hassle of carrying fluid: Race-day fuel isn’t cheap in large quantities. Do you really want to buy pounds of gel every week?
First of all, there is a training benefit to training without any fuel at all. Along with practicing some glycogen depletion and possibly improving your overall glycogen storage, you also will produce crucial aerobic stress that improves your aerobic capabilities.
That said, there comes a point where the losses from a lack of fuel start to hurt you more than the depletion stimulus of training without it is helping you.
Jeff Gaudette once noted that the body can handle about 2 hours of marathon pace running before running out of relevant lower body glycogen. Granted, you’re typically not running at marathon pace for most (if not all) of your long runs. This is a key reason behind most running minds’ stop-loss limit of 2.5 hours for long runs. That’s about the farthest you can go at an easy long run pace without fuel before your body taps out of glycogen and really begins to give out overall.
The suitable middle ground for easy runs is probably near the upper limit of a run’s peak aerobic benefit: 90 minutes. Anything above 90 minutes probably can be completed without fuel, but it may help you more to fuel that run than it may to deplete yourself running without it.
Save for the most hardcore of runners, this indicates you likely will just practice fueling on the weekend long run. Daniels 2Q runners will also need to do so in a long mid-week run (as most of those workouts will exceed 90 minutes), but most everyone else can just fuel and hydrate runs by comfort the rest of the week.
And of course, you will typically fuel and hydrate throughout more aggressive sessions such as speedwork and pace runs, plus should copiously hydrate warmer runs regardless of circumstance. Don’t consider my advice a mandate *against* intuitive fueling and hydration by any means. My recommendation was simply to consciously practice fueling and hydration based on the aid station patterns of your goal pace.