I started doing some local CTC group rides in 1989, as a change from riding around the countryside on my own. Stuck to shortish rides at first, because I was unfit. Worked my way up to >100 km rides, then in 1992 decided to do this 'Audax' ride I'd heard of, that some of those I was now riding with were planning to do. It was called the Dorset Coast . . .
Damn, that was hard! I knew nothing about planning my food intake, & suffered for it, needing a blood sugar recovery break in the afternoon. But some time after the last checkpoint (cafe in Dorchester, where I filled up with beans on toast & a couple of mugs of tea) I found myself riding with someone I'd never met before, & who'd just told me it was his first 200K, as well, & one of us (I forget who) said "We're going to finish this, aren't we?" And it was true. We did. About 8 pm. It had stopped hurting by the time we checked in.
I've only done a few proper 200K audaxes, & none longer (though I've ridden to the start of local ones), but I plod on.