One thing that strikes me about this though (being one of the faster ones on the list), is that the times (including yours B_Kins) are pretty naff compared with 'swimmers'. I swim in the slowest of 4 lanes at my swim club, the others in my lane are mostly in their 70's. Contrast that with my optomistic estimated time (7.00) at my last saw me in nearly the last swim wave.
At a swimming Gala in July I was almost the slowest over 800m, most people were going 8, 9, 10 , 11 minutes something.
On the upside this means there's loads of scope for improvement 