If you have seen a 50% improvement in aerobic pace i 9 months that’s fantastic and you can probably expect to see more gains by keeping your Z2 volume high. I would suggest adding ME training. Your legs are sore after a big day in the mountains probably due to the down hill. You might benefit from doing step downs like I am demoing in this old video. https://www.youtube.com/watch?v=dqK5Xy00BjY. I’m not sure what sort of ME you are doing but I have never seen a mountain athlete of any kind (runner, skier, hiker, mountaineer) that didn’t benefit for using weighted uphill carries as described in this article https://evokeendurance.com/muscular-endurance-all-you-need-to-know/.