Ride more. Take a longer ride a few times a week, but a loooonnnngggggeeeeerrrrr ride. Try doing 5 miles as a start. Then 10, then 20. Work your way up after that. If that sounds like it is impossible you won't know that it is possible until you try it. I told the guy next door to me that I did 30 miles today and he was somewhat incredulous. But I'm 64 and this guy was probably 25. You can do it. Oh, also, see a doctor before you go too crazy.

Do a 50 mile ride and 3 miles to the store will seem like nothing.