I'm 61 and rode a century this fall (into a strong headwind) I never worried about time, just enjoy it. When I do a moderate ride (25-50 miles), I like to get in several good sprints and really burn my legs and lungs. When I go for longer rides (over 50 miles) I avoid sprinting and just keep a steady, strong, moderate pace. I avoid getting aggressive on hills. I want to feel good at the end, not burnt out. That's my approach.