Introduction from southern MS
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!