Vary your riding position by moving your hands from atop the handlebar to the brake lever hoods or drops. This also helps prevent muscle fatigue. Don't stop for more than 10 minutes. Longer breaks can make you stiff and sluggish. Divide the ride into segments.
Some cyclists like to ride a negative split faster in the second half than the first.
Others plan their strategy around a course's biggest hill. Talk to the organizers and other riders about possible strategies. If you become tired, don't think about how many miles remain. Make each rest stop your new goal. Concentrate on your form, and on drinking and eating.