What level of physical Shape is necessary

Here is another question that I can't find the answer to.
I am age 60 and not a jogger; used to be. I backpack Indiana, Kentucky, Tennisee with no problem other than going a little slowly. I hike about every other week for between 7 and 20 miles, camping the night too. I plan to do the John Muir trail in July. Can you tell me what I am in for trying to do this? I won't be acclamated either. I know I have lost 40% of my ability to use O2 being the age I am. I climbed Whitney over fourty years aga from the Portal, straight up in two days and it was HARD! I have backpacked in Montana, Wyoming, California and Colorado, but THIS TRAIL IS HIGH!!!
Thanks for your advice.
(another question about bear canisters will follow if I cannot find one that will help)

Steve,
I'm 64 and a pal and I are going to do the trail in August from TM to Bubbs Creek in Kings Canyon which is going to be about 170 miles in 16 days. There will be +4000 feet days and -4000 feet days. The whole trail is 225 miles or so. Even if one isn't in the best of shape it can be done if the mileage per day is kept low and say 30 days are available for the hike. The first part of the trail doesn't have the major elevation changes so you can get some training along the way. There are many bailout points along the way.
I would give it a try and if it turns out to be more than you are willing to do for days on end then bail.

Here is a link to the profile of the trail:
http://www.pcta.org//about_trail/muir/elevation.asp

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Old Dude

MIKE, THANK YOU SO MUCH. YOU ARE A GREAT HELP. GOOD LUCK, Steve