I'll be going sobo from the border to App Gap at Rte 17 this July. Of the four thrus I have done in the past two were sobo and by the time you get to Middlebury Gap at Vt 125 the rest of way is quite a bit easier. As for doing it in 20 days, yeah it's very possible, but you will hit sections and specific places where the temptation to slow down or stop early will be great. Of course, then there is the weather which could allow you to speed up or make the going slower. The only major work I'm aware of for this season involves the replacement of Montclair Glen Camp south of Camel's Hump. With this seasons high snow pack the mud should be premium grade.