It turns out the charity ride was about 62 miles. Anyway, it was quite painful. I managed 40 without any problems, 50 was fine but the last 10 was just nasty.
I put it down partly to the the weather being so hot. However it’s obvious I’m going to have to do some real training in order to be fit enough to do this. Especially since I was looking to average at least 50 miles a day, preferably more.