How much is too much?

So I decided to swim today. This was an easy decision because Jeremy went to work so I had to go to the gym to work out. (The boys weren't excited!) I entered the pool and realized I was going during water aerobics again. I need to make a note NOT to do that. The pool is small enough. Fortunately I was the only one their swimming laps until close to the end of my workout. I decided to do 3x 20 lengths. Thinking this was a 400 but actually it is less, 340 yards. (No wonder I made such good time, plus the help of pushing off a wall, etc...) I did 3. I didn't know how it would affect me with having been sick and having a race to do tomorrow. Sometimes swimming really wears me out. So I was wondering how much I should do. Then after I got home, I started thinking I should've trained harder because I should really be training for the tri and half not worried about doing well at a 5k. This is when it would be nice to have a coach who understands all this stuff. I was thinking maybe I'll do really well tomorrow because my body will think that I tapered. Ha ha!