Definitely leave it then. You'll be blown away, and totally gutted if you can't do all the bits you want to because of limpy ones....
If I remember correctly, from the car park to the entrance (the famous siq where all the piccies are taken through the tiny crack in the rock) is about 40 minutes on it's own. Seriously, you need to be a bit fit to do it all. For the high up areas, you can have a donkey ride, but tbh, it's not really worth it, unless you fancy a wobbly donkey on a very narrow ledge. Take loads of water too, not many places to get stuff when I was there, about 5 years ago.
There's a bloody good hotel very near too - think it's called Taybet Zaman (phoenetically), built like an ancient village, with a beautiful spa/hammam thing. Was heaven after walking round all day. Also, a lovely hotel Movenpick at the Dead Sea on the way down if you are flying into Amman and driving to Petra.
Oooh, am jealous now. Will think of you next year!