Actually, the more people post about this, the more I realise why I could never really visualise what the forced march 'home' involved, because I don't think EBD fully envisaged it either. As you say, @AlbaAlba, presumably they would have been breaking untrodden snow, if they're cutting off the lake path and heading to a shelf no one seems very sure about routes from or to, but then surely EBD can't have envisioned the CS younger girls and the unaccustomed (and possibly still un-nailed) Saints actually mountaineering, unprepared and without equipment or supplies? So we're back to there being a path of some kind that is clear enough for them not to be wading through deep snow, so people must use the route reasonably often. Which makes me think EBD was just being a bit hand-wavy about it all. 
I do love Rivals, though, and I love that whole episode, with the Saints and the CS girls unwillingly thrown together. (Though having seen people's photos of the 'real' lakeside path, the idea of shepherding a lot of misbehaving Middles along it does make me wonder whether there was really absolutely nowhere the girls could have sheltered, even overnight, while the path was repaired, anywhere on the 'wrong' side of the chasm?)
The other climbing/walking bit in the CS that I find difficult to visualise is the hike they seem to do all the time in the Swiss books, where they head up to the shelf above (am thinking of the beginning of Theodora, but it comes up in several others). I think we're told it's a path between the railway line and a rockface, which makes it sound utterly joyless and suburban (or like one of those dullish 'former railway line' walks where you can't see anything because you're down in a cutting) considering its location in the Swiss Alps, yet it also seems like a strenuous scramble, judging by Ted's exhaustion? Though they do it, from what I remember, in school uniform and blazer, rather than the 'unmaidenly' climbing breeches Eustacia objects to, back in the Tyrol days, though I know that was in winter...
(I don't have any of the books to hand, so could be misremembering.)