This happens to me sometimes too, even when I've taken a break from riding. There are usually a couple of things that fix it for me.

1) Staying well hydrated. My muscles always hurt more and get way more tired if I'm behing on fluid intake.
2) Eat on the bike. If I'm feeling tired early in the ride, I go ahead and eat (I always bring food even if I don't think I'll need it). Wash down with plenty of water. I usually find within 5 minutes, I'm fine.
3) Keep riding. Sometimes it just takes longer for your muscles to warm up. Last weekend I thought I was going to die by mile 8 of my 28 mile ride. But I just decided to push it and keep going. I'm glad I did, because the pain went away on its own and I finished the ride in record time and felt great.

I don't know if any of these are medically sound, but they sure help me.