I have taken folks up the so callled 'Tourist Route' several times and folks have been surprised how demanding it is. They've taken between 7 and 10 hours there and back. It is not a walk in the park by any means.
However top class runners do the Ben Nevis Hill Run from Fort William in under 90 minutes, that is up an down.
A bit of training is needed for the ascent and that is not a stroll round the block at home as some found to their disadvantage.
Hope you make it.
Slainte
Lindsay