I did the 50 mile ride for sports relief 2 years ago. (Manchester). It was about this time that I started gradually building up the miles, as I was doing 22 mile rides up till then. Eventually I increased the 22 up to 26 which I did as a repeat, so I could use the facilities at home as a refreshment stop.
On the day, there were plenty of portaloos on route and 3 refreshment / mechanical support stops. There were marshals everywhere, at all junctions, some roving on bikes and some patrolling segments in vehicles. I had a puncture 2 miles from the half way point as I was walking back, I was picked up and taken back to the support tent. The weather was so cold that my hands were too cold to attempt it on my own.
I didn't eat enough at the half way point or afterwards, so although I made it round, I was too tired to do anything for a while.
All in all, it was a great experience and I wish you good luck with your training and on the day.