The worst long run of my life was a 20-miler I ran on one gel and a lot of confidence. Somewhere around mile 15 the lights went out. My legs still worked, but my pace fell off a cliff, my brain got foggy and irritable, and the last few miles were a slow, grim shuffle back to the car. I'd trained plenty for that distance. I just hadn't eaten for it.
How much your body can actually absorb
For anything under about 75 minutes you don't really need to fuel mid-run; you've got enough stored glycogen to cover it. Past that, the real question is how many carbs you can take in per hour, and the answer is usually more than we bring. A sensible starting range is 30 to 60 grams an hour. The reason you can't just eat endlessly is that your gut pulls in glucose through a limited set of transporters, and once those are saturated the rest sits there and sloshes. The thing that changed my long runs was dual-source carbs: pairing glucose with fructose, which gets absorbed by a separate pathway, so you can take in closer to 90 grams an hour without the stomach revolting. It's why the better gels now print a glucose-to-fructose ratio right on the back.
Start before you feel empty
The other thing I had backwards was timing. A bonk is a lagging indicator: by the time you feel it, you emptied the tank a while ago and you're trying to refill mid-effort, which your body is genuinely bad at. So I start early now, within the first 30 to 45 minutes, and then keep a steady drip going every 20 to 30 minutes instead of waiting for a low. It feels a little odd to eat when you still feel completely fine. That's more or less the point.
Isotonic gels versus the gel-and-water dance
Most standard gels are concentrated, which means that to absorb one comfortably you need to drink four to eight ounces of water alongside it, or it draws water into your gut and leaves you queasy. On a hot day, or a course with aid stations spread thin, that water math gets annoying fast. Isotonic gels are mixed to roughly the same concentration as your body's own fluids, so you can take them without chasing each one with a big gulp. I've been using NeverSecond's isotonic gels for exactly this reason; the texture is thinner and I'm not babysitting my water bottle around every gel. (Any isotonic gel does the same job. That's just the brand that happened to agree with my stomach.)
Train your gut like anything else
Your stomach adapts to fuel the same way your legs adapt to mileage. If 60 grams an hour makes you a little nauseous today, it probably won't after a month of practicing it on your long runs. Which leads to the one rule I'd never break again: don't try a new product, or a bigger dose, for the first time on race day. Every gel, every ratio, every timing plan gets a rehearsal on a long run first. That mile-15 lesson is expensive enough to learn once.
None of this is complicated, which is probably why it's so easy to neglect. But sorting out the fuel did more for my long runs than almost any change I made to the running itself. Worth getting right well before you actually need it.