I'm in my mid 40's and started riding May of 2010 after my knees said no to jogging and have stuck with it unlike other exercise routines. I really enjoy riding and I have a nice park 1 mile from my house. I started out on a used Mongoose MTB, then found a used Huffy MTB and was averaging around 16 mph on the road. I never took in on trails. I wanted to get a bigger frame to match a 34" inseam and found a deal I could not refuse on a barely used mint Felt Z45 with a bunch of upgrades. Now I ride slower to get to the park because the sidewalk is rough in places and the highway is very busy and narrow, but once I get in the park, I average 19.5-20.0 mph for 20-26 miles (depending on how much time I have) if I don't get behind a slow car somewhere during my ride. I ride 4 days per week most of the time.
My take on exercise is my metabolism has sped up tremendously, I have become almost immune to colds, and generally, I just feel better. My knees do get sore when I ride hard but by the next day they are usually back to normal. Now if I was just as disciplined to do some pushups and situps I'd be much more well rounded!