I've done about 10 MS 150s and raised over 440Kin the process. My rides have been in GA and FL. The first one I did, in GA, when I was about 55 was the worst as I didn't train for it. I guess I thought I was still 19! My leg tendons gave me hell after the first day - 80 miles and I only completed 20 on the next day.
After that I trained for about 6 weeks before each ride and had no problems. As others have said, train, plan your nutrition just before and during the ride and pace yourself. In FL it's wind that you might have to contend with. The 150s are very well orgainized. You'll love it. I'm now 66 and wouldn't hesitate to do another one. Depending on wind/hills I average 14-16mph. Good luck.