Thanks! 20 miles up and down MW in one day would be quite a challenge for me! One of the most tricky points for you could be acclimatization. My recommendation is come 2 or 3 days early and camp at Cottonwood CG which is at 10000 ft and do some day hikes to the Cottonwood lakes and get used to the Sierras.
I am also a flat lander like you and always plan to acclimatize before my hikes. For my July 15th hike, I will spend July 14 in Giant forest area (around 7000ft) and start my hike on 15th from Crescent meadows and acclimatize on the way as I wont be reaching MW till July 21. This time planning to sleep in the Smithsonian hut on the last night.
Good Luck!
