Thing is, you need to find something you can enjoy, so it becomes part of your routine, and something you miss when you don't do it. If you have to force yourself to do something, it'll last about 2 weeks, IME.. So there is no correct answer.
I used to go swimming a couple of times a week. That was great when I was so unfit that a half hour session got me knackered
, but got seriously boring when I found myself swimming up and down for an hour or more, and still didn't feel I'd had a decent amount of exercise. Then there's the filthy floor in the changing rooms that always bugged me, and sometimes it was too crowded with kids and nattering housewives swimming in tandem at about 0.001 length per hour. 
Can't abide gyms, so that's out.
Cycling is suiting me at the moment, but not doing enough now the weather is less amenable. Should be improving soon, though.
There you go Chris, eat lettuce off your wife's naked body before meals, you'll be too knackered to eat anything else!

Oh, do be sure it doesn't go limp, there's nothing worse than a limp bit at times like that!