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.
Drumnadrochit to Fort Augustus (21 miles, 2,727 feet climbing):
Climbing for a couple of miles up from Drumnadrochit through the woods; then a quiet single lane tarred road.
Mile 3.7: Follow signs off the road to the right, to Loch Ness Clayworks and Cafe for boozy cakes and drinks. After the cafe, don't return to the road, but part-way to the road follow waymarks and a gate on the right for a narrow path winding up and down through the woods, then more gravel forestry tracks. A few gates to open and close, but no stiles.
Mile 6.8: Choose whether to follow the Great Glen Way: Day 2 - Low Route
or High Route. There's a board describing the sights and route for each option, with distance and elevation for each. Both involve a similar distance and amount of climbing, apparently the Low Route is not as steep or narrow, and doesn't have such big views. We chose the High Route, for the panoramic views. The trail quickly narrowed, getting much steeper. If you're carrying luggage you'll need to push in places, especially up and over a short rock garden.
Mile 7.5: Panoramic viewpoint, and stone shelter. Take care, the 1.5-mile descent is gravel, twisty and steep in places, with rain gullies. We rode part of it, but walked the treacherous sections.
Mile 9.4: There's a hand-made wooden Troll Bridge, apparently inhabited by trolls. More relaxed riding as we climb again through woods and onto open moorland.
Mile 10.2: Great Glen Way
circular wood and stone sculpture forms a perfect picture frame for the landscape.
Mile 10.8: We followed the High Route option on the right, with steep steps to carry up. Keep straight ahead to avoid the steps, the two trails join at Mile 11.5 before a fast switchback descent to Invermoriston; watch out for loose gravel on the bends! The path to the Invermoriston Waterfalls is behind the toilets, in the large visitor car park. Take a look at a ruined old bridge that used to cross the river. Head along a quiet lane and then paths climbing through the woods, then undulating forestry tracks with great loch views.
Mile 14.2: We chose to stay Low: the Great Glen Way: Day 2 - High Route
trail here was seriously steep, about +700 feet more climbing as our High/Low combination, over the same distance!
Mile 18.5 Beautiful forest trails, with a pine needle carpeted descent to Fort Augustus. Head through town to the Loch Ness viewpoint at the entrance to the Caledonian Canal for a selfie. Then choose from various cafes, a chip shop, and pubs for refreshment. We enjoyed perfectly cooked steak and a kebab at The Moorings.