If your fitness regimen is a little less, well, regimented, in the winter, join the club. It's our mammalian nature to slack off a bit during the cold season. In fact, experts say that taking two weeks off of a training schedule or dramatically cutting back on the amount and intensity of it for two to four weeks after competition is important—especially if there's nothing on the race schedule for three to four months. But now that spring is merely weeks away, it's a good time to start thinking about your re-entry plan. Fortunately, there are a few simple ways you can tweak your fitness…