I've been there as well (early 20s). I think I went too far in the gym and as a result of anaerobic exercise my blood pressure kept rising and pulse went up. I don't remember exactly how high it was, but it was a shock for me. Then one summer I started road cycling more seriously. Initially I figured I would do it just once-twice a week to put some aerobic in to the schedule. Well, half a year later I couldn't force myself into the gym any more...The freedom on the bike and fresh air are just unparalleled. Plus, when you climb somewhere, where you haven't been before, you'll really feel that you have achieved smth.

+1 to brbking
I have a chuckle at the guys standing in front of the mirrors in the local gym as I ride past it every other day ;-)