I seem to do a majority of my hikes either in Wy, Co, or the Sierra, the one thing I can say with certainty, coming from sea level in VA is...build in enough time to acclimatize, maybe 2-3 days if you can.
Hike up to say, 10K, then sleep low, repeat and/or hang around as long as you can at, say 7-8k, then head out.
Sometimes you get can away with only 1 day acclimatizing, as I do for my WY trips, but I'll stay at right around 10K for the first few days, going slow, drinking a lot of water, getting enough rest, keeping my fingers crossed, lol.
The JMT FB pages deal with this A LOT. There are also several FB pages dealing with ONLY acclimatization, dealing with primarily the JMT, but great tips for any trip.
Second what Phil said. Get yourself in good physical condition, of course, but being in shapeis not the same thing as being acclimatized to altitude. Time at height is how that happens - climb high, sleep low is good advice. Good fluid intake is important.
And do acclimatize. the illness that can occur are no fun at all
Climbing stair is a great way to build up the muscles and cardio. Something like a 2-3 stories up and down, 15-20 reps (or more!) one or two times a day. Ideally wearing whatever shoes you will be wearing while hiking and along with your backpack with some weight in it.
I'm not at sea level, but still only live at about 1000 feet elevation which is not really any advantage, when it comes to taking hiking trips to the mountains.
But, having made several trips out there, I can make a few recommendations. The first is simply doing as much hiking as you can in places with elevation changes before you go to get in shape.
That said, even being in great shape doesn't guarantee you won't be affected by the higher elevations. When it comes to the elevation in the Rockies, make sure you give yourself a couple days when you get out there to acclimate before doing any major hikes. And, if you can stay at a lower elevation for a night or two, and then go up to higher elevation for a day hike to adjust that is a good way to adjust to the elevation.
While hiking allow yourself more time than you normally would for a given distance, stay well hydrated and nourished, and take more frequent breaks. You will likely notice a shortness of breath, which is normal, but if you start experiencing dizziness, headaches, or nausea you need to descend to s lower elevation as soon as possible to recover; altitude sickness is nothing to mess with.
Apart from being aerobically fit there is not too much you can do to prepare until you get there.
Most people don't develop any serious symptoms below about 8000 feet. You may notice a slight shortage of breath when exerting yourself. If your plans don't take you above that I wouldn't worry about it too much. Commercial airliner cabins are kept at the equivalent of 5-8000ft.
If you are going about 8000ft for very long then first thing is to understand altitude sickness.
Plan on acclimatizing for one or two days once you get there before you attempt any major exertions and have an alternate plan if it turns out you are particular sensitive. You can get a prescription medication, diamoxx, which can help if you know you are sensitive or as a precaution. Otherwise ibuprofen has been shown to be helpful assuming your health allows you to take it.
It is very important so stay hydrated because it is much drier in the mountains. Also sun protection is very important. The altitude exposes you to much higher levels of UV.
Altitude is no joke, so, cudos for wanting to prepare. Great tips here so far here. I'll add ..start drinking water a day or so before your trip. Also, once on your trip, listen to Your body and don't be influenced by the group. Have a plan B (like 2 cars) if you or others in your group need to go back to lower elevation. Lower elevation is often the Only solution. Carbs- often overlooked is that your body metabolizes carbs differently at high altitude so carry quick carb snacks always, and be well nourished especially the night prior to a big hike day. I also carry electrolyte powder or tablets in my pack. I don't use them always in my water because I drink as much if the water is flavored, but break out the electrolytes when the body feels extra taxed.
Before the trip, I try an Altitude Training Mask As a bonus you will look super serious about Covid. Day of the trip, I drink LOTS of water prior to flight and through the entire trip. On the trip, I consider an over the counter boost pill.
*I am not a doctor, I cannot confirm that any of this would work or that it is what is right for YOUR specific situation. These are things I have personally done and work for me.
Hey there! I did quite a bit of high altitude expeditions when I lived in Chile, climbing a few mountains over 20,000 ft high and several over 15,000 ft high. The type of training we used to have was very similar to what Ed Viesturs describes in this article.
As @OldGuyot mentions, each person's body response to altitude depends mostly on genetics, and no amount or type of training can change that. However, the Rockies are not that high (about 14,000 ft), so most likely you won't have any problems, provided that you train for the type of hiking you want to have. Here are a few pointers:
Good luck, and have fun!