I ride at lunch for exercise. I've worked my way up recently to doing 10 miles every other day. Actually I've just done it once (my last outing) but will be the distance I go on MWF for a while. I found that the time it takes for me to go this 10 miles is just about right for me to leave my cube, change, stretch a bit, check air pressure, do the distance, get back, cleanup and get back to my desk on an extended lunch break.
I ride a mtn bike frame with town and country tires (smooth, tread is indented, not sure what to call them) and this 10 miles really kicks my tail. I try to ride it as fast as my conditioning allows so as my legs can handle a distance I just ride it faster the next time.
Any lunch time exercisers here?