Well, in TWS, where she spends a lot of time reminiscing about the early days of their relationship, they apparently spent every weekend climbing with ropes in the Peak District, at his instigation — so presumably he had a strong head for heights in the past?
But I wouldn’t look for logic in Landlines. An experienced walker, she doesn’t try out her new boots in advance of a tough LD walk, they shred her feet, then her size is mysteriously unavailable in both boots and socks at any shop they try, so she buys a size too big and hurts her ankle stepping off a rock, while Moth, who she’s pushed to do the path after his health deteriorates, after an initial wobble, is effortlessly doing long days on tough terrain while Raynor suffers agonies and clearly wants to go home.
Meanwhile, Moth keeps saying ‘Oh, I’m not ready to stop walking yet’ and discovering some other trail that takes them roughly in the direction of Cornwall from the north of Scotland… And there’s a few vague meditations on Scottish independence and Brexit (which is apparently the reason for boot shortages…)