As I'm planning on upping my triathlon distance next year to a 'Sprint' which is a 400m swim I thought I'd try and swim 400m non-stop. Managed it pretty easily although the pool was quite busy so had to weave in and out of a couple of people. Did it in 10:45 so that's my benchmark for when I start having front crawl lessons later in the year. 
Also did another 400m (2 x 200m) so 800m in total which is the furthest I've ever swam!!
So in the last 4 days I've cycled the furthest since I was about 17 (when I did a 100 mile round trip to Wrexham and back and went swimming while I was there), run the furthest I've ever run and swam the furthest I've ever swam! 