I too did mad things in my youth, like the Manchester to Blackpool bike ride for charity in 1989, although I was nearly 23 at that point.
Mind you, I also did it again in 2009 when I was not quite so youthful!
I stupidly thought "I'll be OK, I don't need to do any training beforehand, just a warm-up will do...." (and indeed I did), but afterwards, I wish I'd done some!
And what amazed me, just like in 1989, were riders that were crossing the finish line, then turning round and heading back to Manchester!
I can't remember the exact mileage in both cases, but it was something like late 60s, early 70s.
Since 2009, I have put on loads of weight, but have started losing it while in hospital.
My aim is to lose more weight and try to get back on the bike. I'm not sure if I will be able to with the ankle, but in that case I might get a trike, or maybe fit stabilisers, but I'm not sure if anyone make adult sized versions?
Maybe even hire one that you pedal with your arms, although, I have ridden one of those and they are hard work. Can't imagine doing 70 miles on one!
But one way or another, could I do the BBR again in '29?