It is hilly! A mountain bike is ideal, but a hybrid or rugged road bike would be ok with high volume grippy tires and low gears. The High Route paths are steep, narrow and loose-surfaced, but well worth the effort.
Weather here can change quickly, so carry good waterproofs and warm clothes (it rains here regularly, and can be windy). In warm, calm weather there are likely to be problems with midges; they are less of a problem in early Spring and late Autumn.
For our trip, we left our car in Inverness and used the 'bike-taxi' from an Inverness-based company Ticket to Ride
to carry us and our bikes from Fort William back to Inverness after the ride. Very effective, and well priced. They also offer baggage transfer, if you're prepared to pay to ride lighter.
This is a High Route alternative as part of the Great Glen Way: Day 2 - Drumnadrochit to Fort Augustus
trail. There's a choice at Mile 14.2 of that route whether to follow the Low Route or High Route.
Based on the trails before and after this section, I anticipate that the High Route will include much steeper loose-surfaced gravel forestry tracks and twisty paths.
We chose the Low Route which involves similar trails with several climbs and descents, but is less steep. If you're carrying any luggage, you'll need to push in places on the High Route.
The High Route and Low Route rejoin just before the fantastic pine needle carpeted twisty descent into Fort Augustus.