Just put in some miles. If you can get to the point you can do a 30 mile ride easily 68 will be no problem. And figure out what your nutrition/hydration. As powers2b said, its very helpful to keep a continual flow of food and water into your system. There are some foods and sports drinks I can stomach on long rides and there are some that I can't. For instance, the HEED sports drink mix I simply can't get down, the same for gels, but Gatorade G2 and Clif Shots bars I find great. It's all about personal taste.