I'm also 46 and do about 15-20 mile round trip (I tend to take longer rides in the afternoon to relax; the shortest distance between my office and my house is about 7 miles). There are rumors that our office will move soon and the commute will be about 25 miles round trip. I timed myself yesterday and did 11 miles in 44 minutes for an average of 15 mph on a hybrid, uphill, against wind and in traffic, so I was pretty happy with that. I plan to cycle for the rest of my life; we'll see how that goes. One reason I think I might get away with it is that I really don't push too hard for speed most of the time. A friend from the office does; he's my age and he's always complaining about his knees. I try to to push beyond what I can do over a long haul.