Oh yeah, exercise improves my mood too! I also have a knee problem (and a back problem to go with it), so for me it's only a gentle walk, as recommended by GP and physiotherapist.

But if the sun comes out (or when the rain stops) I'm like "... darn it! now my excuse to sit on my buttocks is gone! ...). I think I'm getting a bit lazy, but I find it hard to drag myself out of the house sometimes.