Hi Jennie
Great question – but almost unanswerable! Unless you can persuade somebody with an accurate trundle-wheel or GPS to walk continuously down the centre of the entire route without stopping or deviating for food, sleep or calls of nature, you have to accept that all figures are approximate. The larger the scale of the map you consult, the more wiggles and bends the route seems to have, and the longer it appears. It's very hard to measure route length accurately.
The figures we give in our guidebook (Table 1, page 6) for a five-day walk (Fort William, Gairlochy, Fort Augustus, Invermoriston, Drumnadrochit, Inverness) show section mileages of 10, 13, 18, 14 and 18 miles respectively (16, 21, 29, 22 and 29 km), with a total of 73 miles/117 km. These were supplied to us (back in 2000

) by the Great Glen Way Management Committee and we don't actually know how they were measured, but we suspect from a GIS system held on computer at large scale.
Your post reminds me to ask the present management for an update. Footprint mapping is normally reliable, and the team has a consistent method for deriving their distances. An extra 4 miles is 5% more than 73 miles, and worth investigating. Thanks for your question!