You know you need to exercise. I know that I do. You know that you need to run lots of errands each week.
Why not combine the two?
Riding your bicycle or walking to that place will take you more time. But you have to go there anyway! Why drive to the gym and run on a treadmill when in the same time it would take you to do both, you could save fuel, help the environment and enjoy being outside?
Disclaimer: I realize this only works if you live in a town big enough that you can do your errands close to home.