Altitude will def. affect your breathing and altitude sickness can kill you.
You'll be limited to riding that atv on open roads and trails, you need a sticker for it also.

Wyoming has reduced price cow tags that are a random draw and no PP needed. Good way to get some elk hunting experience before trying the orange army in Colorado.
You'll see more people then elk in Colorado OTC, but may get lucky.
Folks are everywhere these days, miles back from trailheads.

Altitude is a crap shoot as to my previous post in this thread, I lost. I am in good shape and it wrecked me as I said. I never thought about it because I climbed and spike camped the mountains in Central Montana. But the hotel my buddies brought me down to recuperate in was 124 feet higher than big baldy in Montana. Colorado is rough for altitude.....Hydration hydration and when your ready to pop drink another bottle of water. That was my mistake but as bad as it was I won’t go back...

Another thought the only way to describe it was like drowning for six days. Plenty of people are fine though, so don’t be discouraged. I’m just saying take it serious and read up on how to avoid what I went through. Good luck 👍

You will want to rejet your 4-wheeler. Your owners manual should have a chart showing the proper jetting for the altitude.

Get there a couple days before season to get used to the altitude. It doesn't take much physical exertion for me to get light headed the first day in the mountains coming from 680 feet to ~10k.

The 2nd rifle season has been pushed back a week which increases the chance of bad weather. The area my group hunted in 2019 and 2020 got 2-3 feet of snow each time and temperatures touching 0 at times. The first year it pushed us off the mountain early because we weren't prepared. Bring tire chains! 2020 was a burn ban so no wood stoves.

Depending on the unit you may be able to get deer and cow elk tags along with a bull tag.

Altitude and thinner air means your heart and lungs are working harder to produce the o2 and blood your body needs. Get in Colorado a few days prior to allow your body to adjust. While hiking take it easy and take a break often. Climbing a big mountain on ATV too fast can be a terrible combination. Slow is key.

Sportsman's sells personal cans of o2 you can pack.

Altitude sickness is the main killer for mountain climbers. They ascend too fast. Look at Mt Everest.

Stay hydrated take electrolytes. Colorado and NM have similar weather patterns so be prepared for any weather.

There's athletics training mask that allows you to "adjust the elevation". I know a few that use them to train for various competitions.