I have been biking now for about one year and I don't seem to be able to conquer the problem of my leg muscles seizing up during the second half of a long bike ride (for me that is 40-60 miles). I ride a Trek 7.5FX and 7.6FX and the issue that I talk about occurs mostly on the C&O Canal in Washington. This is a flat trail but does tend to get muddy (which adds to the misery!!!). For example, I was out this morning. I did 20 miles and I was doing fine. I took a 20 minute lunch break and then got back on the bike for the return trip. After just a mile or two I started to get the muscle soreness in my legs. Over time it just got worse to the point that I was having to stop every 2-3 miles. Any advice on what I need to do to overcome this would be welcome.