Biking and Symptoms

I went biking today, for about a half hour.  I took it easy, but by the end of it my left leg still felt tired.  After that we did some shopping, and by the time we got back home, my arms felt heavy.

That's something I notice when I've had a hard workout: my arms and even my gut feel like I'm in a high-gravity environment, with my arms and the core of my body being dragged down.  These aren't new symptoms, per se, but with my exercising ramping up a bit, I feel it more often these days.