If the symptoms are below the neck do not run.
You will fail to finish and you can potentially do yourself permanent damage.
Mountain marathons are really hard, there is no easy way to drop out and the weather is likely to be comparatively cold and wet.
Your sponsors will not worry if you postpone, tuck yourself up in bed and plan a different event.
I hate to say it Jack but treadmill calibration is pretty rubbish and 5 min mile pace is unlikely, though not impossible, for a first run.
Enjoy your running; even on a treadmill.
It costs bugger all (typically £100-200) to have your "commercial" race licensed through England Athletics and get your course officially measured. It's only £27 for a club event.
You also get the assistance and advice of an EA event adjudicator who will make sure you don't make all the usual numpty mistakes.
Amazing that the more "professional" the organisation the more "amateur" their ability.
If the event isn't EA/UKA/Run Britain licensed walk, neigh run, away!
Hampshire Downs is where I mostly run and it's beautiful; not too hilly, not too flat, scenic with a great mix of terrain.
One of my other favourites is the North Antrim Coast, great place to run with fantastic scenery and the Portrush ParkRun on the beach is always an interesting experience.
Visit the official Runner's World page
Follow Runner's World on Twitter
Other Natmag-Rodale Sites
Run For Charity
About Runner's World