Your brother will have a better chance at it if he does the entire trail with you, beginning in Denver. Denver is the lowest elevation anywhere on the trail, and if you both start there you will acclimatize for several days before you get into the high country. That is way better than starting in the middle.
I wouldn't get too hung up on training for a long hike. You will begin at whatever rate your conditioning allows and rapidly improve. Just do it.
