I failed to finish 2 100km audaxes due to various transport problems (I'm a "full value" rider, so it doesn't take much for me to DNF), but then finished the Poor Student with a huge 10 minutes(!!) in hand.
And just over 6 months later, I'd moved on to greater things: DNF on London Edinburgh London (1165km out of 1420). So start with a couple of 50km or 100km, and by 2015 you can join me DNF'ing Paris-Brest-Paris.
(DNF==Did Not Finish)