Interesting topic. 2190 miles. 14 states. Takes 5 to 7 months to complete. A lot of rugged terrain. A person would have to be in top notch shape.
Was looking on line about the Appalachian Trail recently. evidently the fastest it has ever been done was 45 days... that is someone in 'top notch shape'.. proper planning goes further than anything else... there use to be plenty of books that explained about every inch of the Trail... there were booklets available that would go thru a 50 to 100 mile stretch.. know where water sources were, CCC shelters etc.
I am sure there are even more nowadays. At Sportsman's Warehouse, I have found plenty of trail books on the Pacific Crest Trail for the local area.
20 miles in a day is not that hard, after a couple days of getting use to it...but that also is planning and not pushing yourself., especially if you are doing it in 3 or 4 day stretches each day. Aspirin or NSAIDs help to relax your muscles, when they lock up.. along with planning and keeping yourself hydrated, keeps your legs from locking up. know where water sources are... and/or have water filtration kit.. there are plenty on the market.
