I done Loch Ness marathon a few years back. Really enjoyed it. Quite an undulating course and a decent hill at 20 miles. The way into Inverness after is great too.
Totally agree with all advice above. Build it up slowly as the months go by then start to taper before too.
In terms of nutrition I’m a SIS blackcurrant gels runner and my snacks which helped me on devil o the highlands last year was flapjacks, salt and vinegar crisps and Banana Soreen (that’s my fave)
good luck and hope all goes well