A couple things:
1) Stretch your legs a little. I find going for a walk, gentle yoga or a short, easy recovery ride helps enormously.
2) Make sure you're properly hydrated and are getting enough electrolytes, especially potassium and magnesium. If you're in the northern hemisphere and somewhere that isn't Florida, Arizona or the like, it may not seem like a big deal because you're not sweating as much, but that may be part of it. There are tons of sports drinks (Gatorade as well as more athlete-oriented like Nuun or Gu Brew, etc) out there.