I just put the summer tyres and gears on, if you can keep up with me on the first 12km you should definitely do the full 75
More seriously, the great thing about these kind of events is you can push yourself a bit and have the fun of lots of people to talk to, no stress of navigating and the feed stations are nice to take a little load off you. If you think you'll struggle to do it, why not try it anyway? Best chance you'll get for a crack at a longer distance, and once you convince yourself you can do it then it opens up all sorts of other rides, mendips etc. from bristol.
Also if you're flagging, you go through Nailsworth half way round the long one, could stop for an ice cream/beer/fish n chips (delete as appropriate).
Usually seen winching away on a big white 29er singlespeed Voodoo.