My 2 cents worth, if I may, is that you should look to do your trip asap.
In 2013, wife and I set out with our van on a 2 year odyssey around Oz. We had house sitters and a good budget and were able to do pretty well whatever we liked. We were both in our 60's but both quite healthy and active.
One of the most glaring things we noticed was the number of old folks that pulled up alongside us at various attractions (especially those that involved a bit of a walk to access the "spot"), got out of their car, walked around the car park, read the information board then got back into their car and drove away.
These people were on the trip of a lifetime but were not able to access the sites due to physical incapacity. I found it sad to talk to them about their inability to experience what they had spent time and money to see.
The crux of this post is to do it while you are young enough and fit enough to enjoy it. There is nothing worse than pulling up at Kings Canyon or Rawnsley Park and not being able to experience what you have come to see.