You know what the absolute worst part of not working out for a long time is?
The first time back. That's when you're slapped in the face with the fact of how utterly out of shape you are, and how far you have to go until you're back at a point more closely resembling where you were a few years ago, when you worked out almost religiously--no fooling-- and still didn't consider yourself to be in good shape.
I went swimming this morning, because that's my exercise of choice. And I started to feel like I was actually doing something, like my body was actually cooperating, toward the end. Tomorrow I'll go earlier and do more.