This is probably more info then you asked for, but here goes.... You could think about renting a
scooter(s). This can be done at the park or an outside vendor, who will be cheaper. This made such a difference to my older husband; he's 77 now and he's been using a scooter for about 10 years. With the scooter we can do 8 to 10 hours in the park. Without the scooter you can cut the time in half. He doesn't use a scooter at home, but for amusement parks it has become necessary. The average walking is about 6 miles a day at Disney. Of course, this isn't constant walking, but it can be tiring for the older crowd.
You do need to adjust to a scooter (outside rental, here) with the Disney transportation and the crowds that will dart out in front of you. If you can get over those obstacles then it's really worth it. Not only does it speed it up going through the parks, they will be less tired as the day goes on and definitely less crabby.
Next, you need to avoid backtracking in the parks. To a certain degree with fastpass plus you may have to backtrack, but keep it all to a minimum. Less backtracking equates to less walking in the parks. Mix the attractions up with doing a few rides and then a show to sit and relax for a while. Again, do a few more rides and then get a bite to eat for another rest. Use parades and fireworks in the same way.
You should plan your day as much as possible. I would start with getting to the parks at rope drop. If you are onsite get your fastpass plus passes ahead of time through My Disney Experience. If not, that's the first thing you should do when you arrive to a park. You are allowed 3 fastpasses per day for one park. If you park hop most people are saying get fastpass plus for the second, later park. Use extra magic hours as much as possible, if you are onsite guests. The extra hour in the morning has always been delightful for us. It provides an extra cushion if you get to the back of the park before the regular guests are allowed in. It will take at least an half hour to get to the back of the park for the regulars guests.
Address any medical issues your parents may have. Some rides say no to heart conditions and some say no to those who suffer back pain. Ask good questions to see what your parents are willing to ride. Up to about 75 my hubby could handle roller coasters, now he doesn't want to do them. He feels funny in his stomach, now. You can get a Disney vacation DVD which will show some of the rides. It may help to visualize to decide which rides to do. U-tube is a good source for ride videos, too.
Depending on how adventurous your parents are will in a large part dictate what they are willing to do. My hubby will still do the Tower of Terror, but that's it. Roller coasters are a total no even Thunder Mountain or Space Mountain. Sometimes, getting in and out of a ride can present a problem if they have arthritis or similar issues. This comes into play with boat rides and coasters. A few rides have a moving sidewalk like Haunted Mansion, Buzz Lightyear, and Spaceship Earth. I would give a helping hand to whoever needs it. You can be in front of your parents and maybe your hubby could be behind them. Then, they can get help in and out of some of the rides.
The rides my hubby enjoys the most are Buzz Lightyear, It's a Small, Small World, the Riverboat, the MK train, Pirates of the Carribean, Spaceship Earth, Soarin', Test Track, Toy Story Mania, and the Safari. This doesn't mean he doesn't go on other rides; these are his favorites. Use their experience with other amusement parks. We were avid Six Flags guests and did a lot of rides, but the years make us go on less rides, now.
The parks for us are best in the morning, followed by evenings and the afternoons are the least favorite times there.
Ultimately, I would not force your parents into a long day. If they are basically homebodies it will be an effort to do Disney. My hubby and I enjoy going on vacation and we are pretty active through the years. Some like reading books, doing the computer and watching TV more than going out.
