In reply to MarcusP:
I love going mountain biking on my CX bike, but for WHW in a day an MTB is much better and will allow you to enjoy the descents far more. Quite a few people tried it the day I did it and many pulled out because they they ran out of tubes AND patches. I had tubeless LUST tyres on Stans rims and didn't get any.
It's pretty rough for much of the way so have a bail out plan in case you get a terminal mechanical.
The top of Loch Lomond is awful. If I do it again I may take the ferry across and ride up the road, or at least carry trainers to wear on the long rocky walk.
Hope you have a great day, is a fantastic ride. Cheers, Ross.