I once did a 400, after 300km of which I got home, had a shower, changed into civvies and went and watched younger guy play tennis. Then I hopped back on the bike for the remaining 100km after which I had a nice river cruise (OK, it was the Tilbury-Gravesend ferry).
I rather enjoyed the last 100km, though the first 300, overnight was, well, an overnight 300.
I got my SR 2000 in 2008, I think. The 500 was a DIY 200 followed by a calendar 300 (before we had ECEs). I had all of 20 mins between the 2 and so was the back marker for most of the 300. I di get the shny badge, my favourite. (Actually after last year's 24 I could get another but not bothered).