50 miles @20mph is two and a half hours of fairly high intensity effort. I might just get away with that much time without a refuel but it's getting close to the bonk-zone for me
I'll do a 50 mile ride tomorrow but the difference is that it'll be hilly so it will take me three and a half hours at least. For that much time at fairly high effort I will definitely need to think about food. I'll take some flapjack and I might have a stop after 30 miles or so for a café if I feel like a break
I don't think miles is the key thing to think about. It's time spent at whatever intensity level that matters.