as said, food is critical and vital. ALWAYS have breakfast and don't leave for a ride on an empty stomach either. if your body has no food to burn it will start eating your muscle is what i have heard
a cheap second hand hybrid with flat handle bars and tyres more suited to the road would make a world of difference, been there, done that, got the t-shirt!
i went from a cheap, terrible mountain bike to a cheap horrible hybrid and now i'm on a lovely not so cheap road bike. there have been times on all of them where have had to take a rest from cycling and when i come back 10 miles feels like sh*t! when i started cycling properly less than 2 years ago 5 miles was achievable and 10 was a push, autumn last year i did my first 100 mile ride with about 5 or 6 stops. if i were to put the miles on now for the next week or two flat out i would be happy to do 100 miles with 2-3 stops.
yes i am 16 and fit as a fiddle generally and have always had incredible stamina but it's not impossible for most people, and then there are some super yclists on here that will do 100 miles with no stops!
Cheers Ed