I am not over weight, but at 75 I am in relatively poor shape. Several physical limitations. I do have to ride a mule to get where I hunt and I am sure past the age of humping an elk out on my back.Not that I ever did. That being said, attitude is far more important, A person needs that to be able to push themselves to get the job done, I have seen fat guys, skinny guys, sickly guys and healthy guys, guys with prosthetic limbs, all get their elk. It's the whiners / complainers that have to stay in camp after two days that fall short.
As for firearms I can take just about any firearm legal for elk with proper load suitable for elk and go kill an elk with it assuming it hits where I point it and have seen a lot of guys kill elk with a lot of different loads.That part of it sure isn't rocket science.
