Besides what the others posted, the best way to avoid waiting in line I to use Fastpasses and a good touring plan.. Many people recommend easywdw.com or touringplans.com
Even if you don't follow a plan exactly, it will help you to be in th least busy part of the least busy park.
Guests with visible mobility needs ( wheelchair, crutches, walker,
ECV) will be directed to the accessible boarding area. Most of the lines are Mainstreamed, which means that the majority of the line is accessible to all guests nd you will be using the same line as everyone else. The few with Handicapped entrances, like Spaceship Earth that was mentioned, don't need a GUest Assistance Card to access that area. Guests are directed there by CMs when they see a mobility need.
With the exception of a few attractions like that, most do not have a place to sit nd wait unless the guest has a wheelchair/ECV or rollator with them.
Other posters mentioned the distances - that is very important. Most guests walk between 3 and 9 miles a day at WDW, so anyone with difficulty walking will have difficulty at WDW. A Guest Assistance Card doesn't shorten the distance walked.