The official C2C route is fine on pretty much anything, including road bikes, except for the marked ‘alternative off-road routes’. They make up very little of the route, and even sections of these are rideable on most bikes, but parts of them do turn into proper off-road. You’d manage alright on Marathons though. You’d only begin to struggle if it got really muddy, but the sections are so short it’s not gonna ruin your day.
The pass over Bolt’s law would be a little tougher. If you cross over from before rookhope (climbing up the old c2c route and cutting across), which would be the rational option, it can get very muddy across the moor on the way up to Bolt’s Law (Think knee deep). Depending on the weather the fattest of nobblies might not handle it… But it’s not a massive distance. Once you get to the top, most of it is reasonably armoured on the way down, with only small patches being difficult in bad weather. A little volume in the tyres will add to the fun, as it’s rocky in places. …I’ve ridden it more than once on slicks…it’s the high pressures in the tyres that murder you. It will definitely add a bit more work to your journey, with at least one more hill to climb, but if you’re feeling up to it…