Sounds like you’ve made some gains with that plan. Congratulations. I’m glad to hear that, but chances are you’ve just scratched the surface.
If you have 14 weeks more to train then I recommend going back to the aerobic base building phase for 8-10 of those. You can just never have too big of an aerobic base. It will make the next cycle through ME (4-5 weeks) make you even stronger.
As for intervals: Yes, they can make you move a bit faster at lower elevation. They will increase the aerobic capacity of some of the FT fibers. They will also improve the lactate shuttle mechanism. Both of these can be really important qualities to adapt to for higher speed events like a lower elevation SkiMo race or 25 mile trail race. However your event is going to occur at 6000m and there is not enough O2 for anyone to utilize those FT fibers and if you are making enough lactate to tax your lactate removal system you are going too hard and will fatigue in short order. The reason we keep harping on the rather pedestrian aerobic base training is that this is the system that better be doing the vast majority of the energy production for your ski trips.
Ueli Steck and David Goettler are just finishing up a 3 week training camp in the Khumbu region in Nepal. They’ve been running and doing some easy climbing. David uploads his HR data to Training Peaks daily so I can see how hard he is working. He and Ueli just beat their previous round trip record on Island Peak (around 6000m) from Chukung and David’s average HR was 125 for the full 6 hours. That is a good 30 beats below his sea level AeT. These guys work to maximize their low end aerobic capacity because that is the motor that have to depend on at altitude. The more capacity that aerobic system has them faster they will move at altitude. it works the same way at sea level really but it is even more dramatic at altitude.
Keep putting that money in the aerobic bank account. The bigger that balance when you start the ME workouts, the better their effect. And the less chance you have getting writing a check you can cash in your trip.
I hope this helps.