A lot of it is mental. There's been studies that shown that if you simply giving someone under exertion something sweet tasting (but with no calories), it will

A lot of it is mental. There's been studies that shown that if you simply giving someone under exertion something sweet tasting (but with no calories), it will still boost their performance. That study was done on pro level cyclists. I think you do have to replenish eventually. Around mile 20 for me but I think it depends a lot on your pace, how you've trained, and overall distance. Gels and chews are very compact so it's easy to carry on long runs.