I have done it twice.
The first time I was 62yo and we did it as a day hike from the Portal. Started at 4am. it took 14 hours. The second time it took 17 hours. I use a lot of water so I brought a filter to refill on the way up before Trial Camp and again on the way down. After Trail Camp there are legitimately 99 switchbacks that rise over 1700 feet in elevation change. On the way down it feels like there are 999 switchbacks, they seem to never end.
At the John Muir Trail junction on the backside you only have 1.9 miles to go. Who cannot walk 1.9 miles you tell yourself. It is the longest 1.9 miles I ever did. It can take 2 plus hours to complete. Very rocky, and you can see the summit but it seems that you never get closer. I am slow above 11,000 feet and cannot keep up to my hiking partners going up or down so I just go at my own pace and that works for me.
Bring more food than you think you need and eat it, even if you don't have the appetite to. Bring electrolytes of some type. Bring a headlamp. Listen to your body, mountain sickness is no joke. A headache, lightheadedness or nausea might mean you are not drinking or eating enough but it could also be altitude sickness. I have tried Diamox but it doesn't seem to make a difference for me.
I am now 66 and getting ready for a 55-60 mile hike on the John Muir Trail at the end of September. The Southern Sierra's are an amazing place.
