Hello "Larry Duff"
I'm hoping to do it (again) this year, at 63. I haven't done it since 2007/my late 50's, before a slight spot of bother with "The Big C" in 2008. But I'm now no athlete, and yes, it's do-able. What I DO do is regular 60+ mile cycle rides (approx twice a week) here in Cumbria - which is no problem, as I took up cycling in 1996. The 'Maracycle' is very well-organised, their breaks are well-spaced, the company is good - just let 'The Boy Racers' tear on past - enjoy the journey there and back, and pray the weather is kind..
One or two more tips:
Get you bike serviced over first, (its amazing how many dunderheads there are on the ride, with nearly-knackered bike components - rusty chains etc ("calculate the odds of them getting home.. Discuss..") I also fitted a SRAM rear-cassette 12:26 to my beloved old '531c' 8-speed bike to help me more on the hills. This year I've also put on a Brooks B17 saddle: my favourite perch - DON'T do this ride on a cheap, 'ball-breaker' saddle unless you enjoy singing post-event castrato, (and rub the old ass with dollops of chamois cream just before the event.)
Last, DONT forget to carry a' multi-tool' + spare tubes + snap-on chain links..and a waterproof (we all got caught out one year, on the 'BorderTrek' in a real Sligo torrent.)
But you know all this already, don't you ?
Good luck.