The fatigue is likely not cycling related. You don't need to "feed" a 15-20 mile ride, no matter how hard you ride it. Period. Thirty to thirty five shouldn't be a problem either. That's maybe 1000kcal at a moderate pace, and you've already got that inside you ready to go unless you're very small or woefully unconditioned. In my case, I usually get pretty sleepy around 3pm, no matter how far I rode that morning. Usually because I sleep barely 5 hours a night, and my diet is borderline terrible. Nothing to do with the bike at all.