The Great Glen is the surface expression of a strike-slip fault about 430 million years old. Glaciers found the line of weakness, gouged it into a deep diagonal trench, and left three long lakes strung along the floor: Loch Lochy, Loch Oich, Loch Ness. In the early 1800s Thomas Telford joined the three with the Caledonian Canal, opening a coast-to-coast inland waterway. The Great Glen Way, designated a way-marked trail in 2002, sticks close to that canal for much of its length. That is why it is so flat. The trail follows the same accident of geology that made the canal possible: a single straight corridor through Britain's most mountainous country.

The official start is the old sea wall at Fort William, near the railway station. The town itself is a working Highland service centre, the northern terminus of the West Highland Way and the base camp for Ben Nevis. From the sea front the trail hugs the coast to Inverlochy Castle, then west to Corpach. There it meets the Caledonian Canal at Neptune's Staircase, an impressive flight of eight locks that lifts boats 19 metres in less than half a kilometre. Walkers climb beside the locks on the towpath. From here the route is level, mostly. Loch Lochy comes first, with the pepper-pot lighthouse marking the canal entrance, then the forest tracks past Clunes and the Clan Cameron museum at Achnacarry.

Invergarry Bridge sits at the southern end of Loch Oich, the highest stretch of the Caledonian Canal. The trail then runs along the old railway formation, now a cycleway, past the ruins of Invergarry Castle, sacked by Cumberland's troops in 1746 and never rebuilt. North of Loch Oich the path threads between the River Oich and the canal itself, dropping through two more lock flights to Fort Augustus. The village is a small tourist town with restaurants, pubs and boat trips. The Caledonian Canal descends here through five linked locks straight into Loch Ness, and the lock gates are usually crowded with people watching narrowboats step down toward the deepest freshwater lake in Britain.

From Fort Augustus the Great Glen Way runs along the west side of Loch Ness, 23 miles end to end. Walkers choose between a low-level lochside route and a high-level forest track that adds a mile but offers wider views. Drumnadrochit, midway up the loch, is the centre of Nessie tourism, with the Loch Ness Centre on one side of the road and Urquhart Castle, the most photographed ruin in the Highlands, perched on a headland in the loch. Beyond Drumnadrochit comes the steep climb to Abriachan and the high cattle drove road through Craig Leach Forest, then a sharp descent into Inverness along the River Ness. Inverness Castle marks the ceremonial end of the trail. It is the most northerly city in the British Isles.

The trail is no permits, no fees, no waymarking confusion. Wild camping is legal under the Scottish Outdoor Access Code. Scottish Canals operates small Trailblazer rest sites with showers. Commercial luggage transfer companies will move your bag between B&Bs while you walk with a day pack. The recommended direction is south to north so the prevailing weather is at your back. From June to August, midges become an inescapable feature of the experience. Repellent helps, head nets help more, and the Wikivoyage authors note that flapping a handkerchief while swearing also works about as well as anything else. Plan well. Book ahead in summer. Bring waterproofs in any season.