I've yet to hike the Quehanna but I am active in the STC club that maintaines the STS trail. We just put together a website that provides quite a bit of info on the trail. It's still a work in progress but you might want to give it a look.
http://www.stc-hike.org/
I've been working on the trail off and on this winter. It looks in pretty good shape but will be getting out in the next few weeks with the other folks to give it a spring cleaning. We just had high winds and know of new blow downs that need cut up but certainly passable.
I'll be hiking the loop May 8th – 16th. I've done it in 5 days but I've decided to take it easy this year and try my best not to return home too soon .
Some thoughts, Black gnats have been bad in the upper part of the loop the last few years later in May. So I am giving it a go earlier this year. Water is not a problem, I would say a source every 5 miles or so. Just two wet crossings I can think of that could be high but not very wide.
If you have any specific questions about trail heads mileage between points etc just let me know. I work just 15 min. away from the northern section. I can even help out with a food drop if you dont want to carry all the food at once. This old guy doesn't any more