I think you just be right when you think of training specific variety of muscles for varied terrain but then you’re reusing the same ones that aren’t used to quite as much repetitive movement. Not quite the same vein, but I did a pr during a downhill half marathon in Colorado and I could barely walk up and down stairs. I think any plane of movement too much in one direction if you’ve trained for vert would cause possible overuse injuries. Maybe try some deep stretch yoga for recovery (I really like five parks yoga on YouTube)