How Much Is Too Much?

I had some leftover energy last evening, so I went to the gym and got on the recumbent bike for 30 minutes, then hopped on the elliptical for 21 minutes. Add this to the 41 minutes I walked yesterday morning, and I think I overdid it yesterday! This morning I'm more tired than I was yesterday morning, so I stuck to the recumbent bike for 40 minutes.