The JMT is a well-maintained, well marked path, with few really rough sections. The full trail is 221 miles. For 13 days, that’s 17 miles per day through the Sierra - or 8 1/2 hours of non-stop hiking at 2 mph - a reasonable number for the ups and downs. Certainly doable, but you will miss many of the unique side adventures.
There’s also the issue of altitude acclamation. If you’re in great shape or live at altitude you will do ok, but as a Flatlander,I find the first couple days distance usually much less than planned because of late afternoon fatigue.
You also might think about why you are doing the JMT. To tick off the miles or to enjoy the magnificence of the Sierra - or both. I spend a lot of time in the Upper Kern and can never understand why the thruhikers blast through the Bighorn Plateau without at least stopping for a while to let themselves be awestruck by some of the most spectacular scenary on earth.
Why not start somewhere other than the Valley?
booger