Many people tend to cycle in a gear that is too difficult, and they end up using brute force to push the pedals around. Called "mashing," this tires your legs out, leaving you with little energy for the run at the end of the race. Learn to "spin" instead. Find shoes that can clip on to your pedals (most bike shops sell pedals that you can clip shoes onto). Another option is getting a cage to strap your shoes to the pedals with. This allows you to pull with your hamstrings as you pedal, balancing out the pedaling action in your quads and conserving energy.
Ride 15-20 miles on flat road. Work on spinning your legs for 60-90 minutes without mashing. Your intensity should be a mixture of Zone 1 and Zone 3, pushing your race pace but also focusing on perfecting your pedaling technique.