Besides the pedaling part, I've found that lack of fluids and food along the way always seems to be my biggest problem. I'll go 60 miles without drinking or eating and then "BONK". Means all of a sudden you are 100% out of energy. Up to that moment, I am always fine. So learn to eat and drink along the way. The gym is fine but I have always been a gym rat, but have found out that there is no substitute for time on the bike. Like some others have said, continually increase your mileage and ride at leat a 50, 63 and 75 miler before attempting the 100. Alot of folks here laugh at 100 miles, but it's a LONG ride......Good luck and start riding today!!!