If I could add 3 pieces of advice:
1: Sort out little issues before they become big issues.
So, if u got a small blister starting on your big toe, stop, and sort it out, before it becomes a huge blister, and potentially ends your race. (duct tape works wonders for this)
2: Arse cream, get the best you can find, and use loads!! I used to supplement my arse cream with a product called Anusol cream. It has a local aesthetic (its available in South Africa, I am not sure about the UK)
3: Get used to riding on regular plain food. Drink water (and maybe a electrolyte every 2nd or 3rd bottle) Stay away form to much the sugars, as you will begin to feel very nauseous, and ultimately puke!! (I have seen this end races many many times!!!)
Also, I have found day3/4 to be my toughest, after that you seem to get fitter, on the ride (or your body learns to suffer?)
If you consider pulling out, don’t ever make the decision at nigh, most problems seem better after a good nights sleep!!! So golden rule, if u want to bail, first make it to the morning before deciding!!!
Good luck.