Can't say I understand the people thinking biking can replace gym time. Maybe if all you do in the gym is cardio, but if you do strength training of any sort then the two are not interchangeable. Commuting to work doesn't do to the body what squatting, bench pressing, deadlifting etc. do.

That being said how about starting slow and seeing what happens? How about picking 1 day a week that is most convenient for you and your family for you to bike to work? I'd imagine that would only add an hour to your time spent commuting per week, which isn't a huge change.