[YMMV]Me and mine (blind) ride a tandem together to her worksite starting zero dark thirty, 4.5mi on a rolling urban course with one significant hill. I ride back light, and may add additional miles depending on the day. I don't think the commute is the time or place to worry about anything other than the details of making good time safely. We do a 3x/wk full body workout with significant weights and add cardio (elliptical, rower, swim) and abdominal work depending on the day. This is so the commute can proceed as it will. We get fit doing anything except cycling so that our cycling can be as efficient as possible. The commute certainly doesn't take away from our fitness level, but I am unsure how much it adds, given the reality of (lack of) ambient light, traffic lights, stop signs, loading, unloading, detours, weather ... ... [/YMMV]