I grew up on a bike so I cannot say how many miles I used to ride as a kid. After a long hiatus I returned to cycling at age 35. Again I wasn't tracking my mileage, but I don't think I could handle more than about 12-15 miles in one ride at that time, and even medium hills were making me turn around and go home. I'd say 40-60 miles per week at first.

It took a year or so before I could do 50+ miles in one ride without feeling wrecked. These days I am in much better form and am regularly racing distances higher than that with good recovery by the next day or two.