I'd gotten up to 40-odd lengths when I got out of the habit of going swimming last year. 20 'bout done killed me yesterday. Stupid muscles. Also, the locker key falling off into the pool was most amusing. How I chuckled as I wandered along squinting at the water for a key, or anyway for a small dark thing that could be a key or frankly it could be a fallen-off plaster how can I tell what with not having my glasses which are in the locker which I can't get into without the key aargh.
(A bat would have been blinder than me, though, for once- I don't think sonar penetrates the water-air interface.)