I tell people that I am running 100 kilometers on Saturday, track the race here, and flying to Nepal on Sunday, and people laugh, or they look at me in disbelief, but really I've been planning this for some time and it makes total sense to me.
First of all, it is necessary to get into some kind of good physical condition before an expedition. Spending two months at high altitude is wearing on the body. You need to go in with strong muscles. People train for an expedition the same way that people train for a foot race, with running and hiking, and perhaps more emphasis on upper body and core strength for the mountain expedition. In my experience training for one or the other is rather similar. Which is to say, when I have been in the best shape of my life it's always been to run a race, and several times I have wondered what would happen if I used that fitness on a mountain. My two previous expeditions I had injuries or recovery from a previous event prevent getting into great shape, but I went in with good fitness.
An expedition is an interesting event, to call it something. The whole trip is planned around being in good shape and healthy after having five weeks or so to acclimate to the altitude. Most of the time is spent hanging out around basecamp reading books, talking, drinking tea, eating, and playing cards. It's what I imagine the life of a professional athlete is like during a stint of altitude training. About once a week we will go up above basecamp on the mountain for a couple nights before coming back down to rest and continue acclimating. Those days are not easy exactly, but the whole time the goal is to take them as easy as possible so that when the summit push comes, you have enough energy to get up and back safely.
My point is, of the two 8000 meter expeditions I have been on I have not had an actual summit day yet, so I've always come home very acclimated with tons of unspent energy that I had expected to use on summit day. Which is a long way of saying 95% of an expedition is "easy" doing hikes and load carries to higher camps (which is not easy, but certainly not something to physically tear yourself apart over) and resting. An expedition is like knowing you have to run a 100k race, but you aren't exactly sure when, and your finishing time doesn't really matter, but you absolutely must be able to finish it healthy. For examples of people that did not "finish it healthy" watch the movie Everest.
So, it seems very logical to me to train for a race, particularly an ultra race, before an expedition and since I know that the first five weeks of the expedition will be "easy" I will have time to recover from the race.
Another way to compare, on the trek to basecamp I may hike up to 12 miles in one day, depending on how fast I am acclimating. I'm about to run 62 miles, in one long morning. It sure makes those 12 miles to walk seem awfully easy.
Finally that gets to the last bit, the mental part of an expedition. I've taken a lot of people into the mountains, and time and again they tell me, "that was harder than I expected." These people have never been ultra runners and I think that makes it harder for them. As I get older my experiences give me more fuel to "run" off of during difficult times. I know that summit day on Everest could be as tough as 4 AM in a 24 hour race. Until September 2014 I had no idea what 4 AM in a 24 hour race felt like, now I can tell you it's dark out, your legs are tired, and most of the other runners are taking a nap, so it's a bit more lonely.
The point is, I'm ready.