I've switched from ultra training to marathon training (recently) and it's a tougher transition than you'd expect. I'm not even at altitude and it was tough to get my speed back.
Ultra Trail Guy, it's gonna be tough for you if you're faster than 2:40s for the marathon. The people here think of hills as a 10mi run with 500' of climbing, but I'm sure you could hit 500' in the first mile alone if you wanted to.
To start, you need to start doing drills and strides regularly. Every other day. Take them seriously too, don't just do them on some singletrack because they won't be as effective. Do them at the end of your run, and do the drills just before to make sure you can really open up your stride. Do them barefoot on grass or on some hard/consistent surface. Not gravel, not a trail.
After a few weeks you should be more comfortable and getting faster with ease. You should then start the actual speed workouts. These could start as fartleks (still avoid singletrack for any quality workouts) and turn into track workouts and tempo runs. Your workouts may not go "well" for quite some time, but realize that you're still getting the benefits even if you aren't feeling fast.
Also, I just noticed that you plan to make this transition from August to November. That's an issue - it's not enough time. What race are you doing in August? Hopefully it's not a sky race or something steep/slow. If you care more about the marathon than that race, you should start doing the faster workouts in June or July in addition to whatever mountain running you're doing.
As for altitude adjustments, just deal with it. You have no solution other than to suck it up. You can still do speedwork, it's marathon training, nor 1500m training. You don't need to do 8x400 at mile pace to run a good marathon.