Don't think of your 6-8 mile rides as the exercise you have to do. That'll put a drag on your goals just as that New Years gym membership does.
Think of where you want to go and get there by bike. Get a map and plan your route checking for bike lanes, safe roads and gentle hills. Go visit a friend, go get a coffee, hang out and get back home.
Next time go a little further.
If work isn't too far away, try bike comuting. Ten miles one way isn't unheard of. The ride in the morning is gentler than the people rushing in their cars going nowhere. You'll have a blast when you ride past them during traffic jams and collisions. The evening ride is a great stress reliever. Some people right here on BikeForums drive part of the way (or bus it) and ride the rest of the way to work.
Don't over exert yourself. Ride at your own pace but get to where you want to go.