Thanks for all the responses.
I’m not worried that I’ve not got the distance in me, but more whether I can finish and not pick up some overuse injury. I’ve got a bike holiday on the MTB coming up in a couple of months coming up and I don’t want to nix that or be less than 100% for it.
I’m not treating it as a race so not worried about trying to beat other folk and such. I just want to enjoy the day, improve my fitness a bit, see what other routes are available around my area and have fun!