It’s about 270 miles or so from South Pass to Old Faithful. For an experienced hiker, that is doable in three weeks. If you find yourself running behind, you can always bail before. The trail in northern Wyoming has some really beautiful sections, including the Wind River Range, with a lot of good alternate routes.
Challenges: navigation in some sections, crowds in the Winds, possibly bears, possible snow in the higher passes, thunderstorms at high elevation.
You’ll need reservations in Yellowstone.
Get good maps. Get Jim Wolf’s book. The ‘official’ route disappears in the Gros Ventre, leaving you stranded. Wolf’s routes are better. If you are feeling really adventurous, some of the cross country routes in the Winds are supposed to be really good.
Ginny