My first book on the Scottish hills was the Scottish Mountaineering Club Guide to the Cairngorms, the introductory section of which included a report of this incident. Lord Kennedy was apparently well-known for his sporting feats and athletic wagers, and this one appealed to me.
It is not entirely certain that he used the route through the Lairig Ghru; but it is the most likely way, and his was a quite remarkable achievement, even if his actual time was around 35 hours instead of the twenty in the song. I think that’s called poetic licence.... Today, the way through the pass is a popular challenge for long-distance walkers, but it remains a serious undertaking, as it takes you through very remote country, and reaches over 2700 feet.
The song is very much in the traditional style, and has in fact been mistaken for a traditional song. Mind you, it’s nearly forty years since I wrote it, so I suppose it more or less is by now.