In reply to crabtreer:
My personal preference is for a thumb compass which you can use to take bearings off like the one in the link below. Using a basic thumb compass in mountain marathon terrain can be a little limiting especially if you need to follow a bearing.
https://www.compasspoint-online.co.uk/product/moscow-3-xl-r-thumb-compass/
Apologies if it's teaching Granny to suck eggs, but a couple of tips: Use the compass in your non-dominant hand so you keep you dominant hand free to rotate the bezel and dib with, and go and practice using a thumb compass before the event -- they can take a bit of getting used to