Hi Jonathan
You're right, lots of sources show the first day as Fort William to Gairlochy (10mi/16km) and you can easily save a day, perhaps even two, from the common idea of six days for 73 miles/117 km. Our guidebook which has just reached its
fourth edition has a table on page 6 which shows a 5-day option with overnights at Gairlochy, South Laggan, Invermoriston and Drumnadrochit: this gives you two 18-mile days (29 km, on the third and fifth days) but is perfectly feasible if you push on a bit. The six-day option splits the middle long day into two, with the benefit of an overnight in Fort Augustus.
If you don't need part of the first day for travelling, and can face a 23-mile first day (37 km), you could even squeeze it into four days: skip the Gairlochy overnight and just stop at South Laggan, Invermoriston and Drumnadrochit (with mileages of 23, 18, 14 and 18). However, this approach is unlikely to give as much enjoyment as the five- or six-day itineraries which allow time to look around.