Friday night's alright for ....swimming?

8 hour swim with a twist
Since September, I've had the ...er... pleasure of completing three 8hour+ swims, all in Source 25m pool, so I should be used to it. Some I have started in the dark, some I have finished in the dark - tonight it'll be both.
It's a charity swim from 10pm to 6am. I have had a tough enough week, but then they're aren't any easy weeks these days are there? I swam this "event" last year, my first ever 20km swim and I remember a serious dip around 2am and when i finished i could barely lift myself out of the pool. I could barely lift my left arm, had serious chlorine lung and shouldn't have driven myself home i was so bloody tired! It will be interesting to see how i fare this evening.