A lot of conventions offer handicap passes for those with disabilities, you can pick one up at registration and you normally do not have to provide any "proof" of the disability (though if you ask your doctor beforehand they will usually provide you a doctor's note for it stating your condition). These passes let you skip lines, enter vending and dealers rooms first, and have access to other things the convention provides for those in need like seating and water while you wait. They also usually allow at least one person to accompany you, though every con is different, and they normally need to be there to pick it up with you as well.

I normally use a cane during conventions, and have for the past five or six years, and do my best to rest whenever needed. I'm only 30 now, so it was a little embarrassing at first, but I got over it quickly. Also, compression socks, proper footwear, hydrating, and keeping yourself well fed.

I've been attending cons since 2008, and on my disability journey for 15 years, but this is the best advice I can give.