I've hiked part of the John Muir trail. According to a book I have, Hiking the Big South Fork
, it is 50 miles, but I think that only includes the northern part. There is a gap in the trail and another section farther south. With all the trails in Big South Fork and the Sheltowee Trace going through the area, you could design a much longer hike.
The National Geographic Trails Illustrated Map of Big South Fork
has all the trails on it but doesn't have mileage. The book has mileage, but only for each hike described in the book, so you have to work at it to get mileage for your own hike if you don't stick to exactly what's in the book.
It's a nice area. I definitely recommend it. The rangers we talked to there had hiked some of the trails, but not always recently, so they may or may not be helpful in planning your trip.