I read somewhere that your aerobic performance won't really degrade for up to 2 months after you suspend training. I've noticed many many times I come back stronger after taking a week off. When I was weight training, it was real easy to see as I simply could lift heavier weight. If I ever reached a plateau in my training I learned to simply take some time off and I usually surpassed it when I got back.

The body doesn't get stronger from training, it gets stronger from resting after the training....