I have never attempted to ride a brevet on a MTB, although several members of our club here in Mexico have done it. The longest distance a few have completed is 400 Km. By the time the 600 Km rolls around, everyone is on road bikes.
Regarding wheel size, it absolutely doesn’t make any difference. I am one of the few ones with a 26” wheeled rando bike. I have run road tires from René Herse and Schwalbe between 1.5” and 1.75”. I’d have to say Schwalbe Marathon Racers are my personal favorites - they roll quite fast and offer excellent puncture resistance on a timed event.
My bike in rando mode:
In gravel bikepacking (touring) mode with knobby tires:
And it took me safely over the finish line and within the time limit at PBP 2015
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