Sorry, I type too fast and my brain fills in the missing words. But yes, I did the swim, she did the comment. It happens all the time, we tell someone we did a twenty mile hike (not uncommon) and the assumption is always that's we're doing it for charity. It's only a matter of time before my 10 minute amble to the train station is deemed worthy of sponsorship.
As for the swim marathons, things have softened up, I did 400 lengths back and forth without a break (well, you could stop at the ends for drink and pause, but simpler to keep going). I will say that my daily 50 lengths vary – sometimes I vague-out and it's all over, other times, it's a grim tick-tock of each and every length. Those are usually the days when I swim like a sack of spanners. Generally, after many years of swimming I don't really tire beyond a certain point unless I really thrash, so the limit is boredom. That said, I often do 50 lengths in the Crystal Palace 50 metre pool just because. It's easier to zone out when you're not having to turn all the time.