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M-J , you must mean the "Loch Ness Marathon" ? Right ? If you do, the hill at 19 miles ( just outside the village of Dores) is a decent climb and one of the steepest I've encountered in a road marathon. There may be tougher climbs in something like the Snowdonia Marathon. Anyway it is steep and quite long and comes at a point in the race when you may be begining to feel tired anyway.
There are some steep downhills earlier in the race that are a bit "quad sapping" . But overall it is a great race, fantastic scenery and I'd recommend it