The spring at Little Jimmy, two miles from the trailhead, is the only water on the hike. It can be a long, dry walk, especially on a hot summer day.
It's more like eight miles between Islip Saddle and Baden-Powell.
I'm not sure I would recommend it as an overnight backpack, as the only water is at Little Jimmy Spring. Even the Islip-Baden-Powell-Islip round trip can easily be done in a day. My wife, who has little hiking experience and only modest conditioning, was able to do it in about 8 hours, including lunch breaks.