Swimming on tired legs - round 2

I headed out to Titlow yesterday. I must have been a little stinky. There were 7 people swimming laps when I walked up. After 5 minutes, there were only two others left. The main set was four sets of swimming where the distance decreased and the the speed required increased. It didn't matter how much harder I tried, my gearing selection was very limited. My clutch would not shift into fifth. It was still a solid 4.5k swimming. I was exhausted at the end. Afterward, I stopped to let the dogs play for 20 minutes. I wanted to run later in the evening, but my energy levels were non existent. I stayed on the couch and acted like a potato.