Walking, running and biking are all fine, but they build a different kind of muscle than weightlifting. They're also potentially boring as hell.
I have to recommend finding a decent personal training and start power lifting. I've been doing it for a few years now and sometimes walk past a mirror and don't recognize myself. (Of course, the newly bald head makes my self image even more out of whack.)
I see your profile says Baltimore, MD -- not too far from York, home of York Barbell and tons of power lifters.
It isn't cheap to get a trainer and is a much different kind of commitment than running/biking, but with literally 2 1-hour workouts per week, you can reset your metabolism, lose fat, build muscle and look like a totally different human in like 3 months. I'm pushing 40 and am in the best shape I've ever been, even when running 60+ miles/week in high school.
But seriously, doing anything is better than doing nothing