I used to cycle to work (about 3 miles away from my place) and after 6 months I still felt absolutely cooked after the uphill trip back, and the inclination was

I used to cycle to work (about 3 miles away from my place) and after 6 months I still felt absolutely cooked after the uphill trip back, and the inclination wasn't even that accentuated. So unless you live in a cold and flat-as-a-pancake area there's really no way to classify 15 miles as easy for anyone but seriously fit people. Even then, it's not something a non-athlete (amateur or otherwise) can just pick up in two weeks.