Swimming on tired legs

I wasn't sure how my swim was going to turn out today. My legs were tired this morning just walking around. The workout was 4.5k with the main set of 400, 2x100, 200, 2x100 on a 1:25 base send off three times through. My pacing improved each set for the 400. I am gonna have to start working even harder, if I want a chance to beat Lisa. She was only 17 seconds slower than Karen at their race yesterday. Watch out Lisa, I am gunning for you!