This thread isn't what I thought it was going to be about.
I will present the answer I prepared anyway:
I'm in the best shape I've been in in about a decade because I went back to mountain biking for fun & fitness this spring and have been going out about 2-3 times a week since.
I've always biked for local transportation, but you don't necessarily get big cardio or strength benefits from that if you're lazy like me and use the lowest gears to always go slow up hills.