Good luck!
I guess my last 100 mile ride was early '80s too (Bristol to Taunton & back) when I was a teenager. No chamois cream for me back then - I was wearing an ordinary pair of jeans and a cheap pair of trainers. Looking back on it I'm not surprised I bonked only a few miles from home as I took very little to drink or eat with me, but my gran did feed me full Sunday lunch when I got to their house in Taunton!
I'll be attempting 100 miles again at the
Wiggle Mendip sportive in August. It only just occurred to me that both will have been on the Mendips - I might even end up riding a small stretch of the same road. Back then I went down the A38 all the way.
Do report back how you get on.