Great Glen Way
The Great Glen Way is a 73 mile (118 kilometre) hike between Fort William and Inverness, in the Scottish Highlands. It follows a major geological fault that runs east-west across the width of Britain. The walk takes you past the famous Loch Ness, where you can scan the waters for the Loch Ness Monster.
It is generally recommended that you walk the route starting at Fort William and ending in Inverness, because this direction means you have the sun and the prevailing wind at you back the whole time. The walk passes through Gairlochy, Loch Lochy, Laggan, Fort Augustus, Invermoriston and Drumnadrochit. There are also diversions along the route to the Commando Memorial at Spean Bridge, and several former castles/museums.
There are at least some accommodation options in each of the towns that the Great Glen Way passes through.
There are Tesco outlets at Fort William, Drumnadrochit and Inverness. Apart from this, if you need to buy supplies along the way you will need to plan ahead and pick them up at the first opportunity. Invermoriston has a small convenience/grocery shop. Fort Augustus has some good cafes and supermarkets.
Maps and Guides
There are several guides for purchase, and at least one comprehensive website for the Great Glen Way, but it is possible to walk the route without a map (there are route markers at all key intervals) and without accommodation bookings. 
The Great Glen Way has some fairly strenuous sections in which you are walking into or out of the valley, but most sections can be broken into 11 mile parts. It should take 4 to 6 days to complete. The official end/starting point of the walk is the Castle at Inverness.