It's a good idea and of course the logical extension of saving fuel is eventually not driving and riding a bike or walking instead, but that said, there are several good existing services that do that already and have been for years. We were even inspired by them somewhat.
We Endure is a simple exercise tracking site for logging your swims, runs, and rides. Lately I've also started using
RunKeeper, which works with iPhones and Android devices to track your workouts via GPS.