You're ight that multiple complex variables come into play. For example, it you are commited to bike transportation you're likely to plan to live close to work and shops.

If you are out of shape and start biking for fitness, you may end up living longer, and as Professor Ulrich points out, that may actually lead to greater lifetime energy consumption, because only a small part of our daily power usage is for transport, but if you combine it with other environmentally sound choices you can counteract that unintended consequence.