Exercise presents a perfect opportunity for a guy to get some officially sanctioned alone time. The best investment my wife and I made when I was staying at home was a family membership at a gym with a daycare. On those long, cold, Chicago winter days when my son and I would otherwise be confined to the house, the gym became my second home. I went four or five times a week, got into the best shape of my life, and made my days shorter.
Not everyone has the luxury of this kind of schedule, but the point is that I developed a habit. If you want your partner to exercise more, you have to give him an opportunity to develop that habit. I definitely wouldn't have started working out as much if my wife and I hadn't gone in on that membership together. I would have felt guilty about spending the money and dumping my kids at the gym's daycare for two hours a day.
Your partner doesn't have to go from couch potato to triathlete either. Find ways to exercise together. Plan weekend bike rides. Sign up together for a 5K to give yourselves a goal, or join a co-ed softball or soccer team.