In reply to WaterMonkey:
> When we walked the route Strava logged it as 43km. His log shows just over 35km. The route down from CG was different but can't imagine that would shave 7km off
Not sure if you counted ascent of Snowdon or walk off afterwards, but 35k is the usual summit to summit distance (21.5 miles quoted in the 1997 Clayton and Turnbull book).
Looking at the Strava blog post he seems to have done Donnelly's CG descent, back tracking from the (true) summit on the ridge to Bwlch coch and down into Cwm uchaf, (rather than continuing over CG to the North Ridge descent).
Post edited at 11:22