I'm getting back into biking as a gentler form of exercise in comparison to running, which has become more difficult ever since I turned 60 two years ago. I have a Trek 7500 hybrid and a Trek 4300 mountain bike [which is a bit small for me (purchased for one of my sons a few years ago)]. I'm wondering what type of bike is best for exercise. This would mostly be on roads and bike paths, but I also enjoy going off road onto not too difficult trails. I've been thinking about a 29er HT. Rode 23 miles yesterday on the 4300 at an average pace of 13.9 mph. The 7500 is slower, I believe because of the more upright position, the softer front shock, and not as good a weight distribution over the pedals. Any comments on this would be appreciated. To learn more about me, visit my web site at http://www.tarrh.com.