Why Is This?

It takes months to get into shape but only a couple of weeks to get out of it. I did my usual BodyPump class on Monday after missing two weeks (one at Lotusphere, one with the flu) and my legs have hurt all day today. I'm going again tomorrow because I know that although they will hurt, they will actually hurt less after I work them out again. It's really annoying though that it only takes two weeks to undo a lot of prior work.