For what its worth, I have drank (grammar?!) from the spring at Guyot many times the last couple of years, without treating it, and have never had a problem, BUT that's me, and its one of the few places I do so. Otherwise, the drop down to Zealand Falls Hut via Twinway is a pretty unrelenting downhill. It says 1.2, but feels longer. If you do so, I'd say leave some time to go up Mt Tom on the way out (to Highland Center I assume is where you are leaving a car?) because its worth the quick jaunt for a nice view. I haven't descended the Zeacliff Trail in a dog's age, so my memory would be fuzzy on comparing its downhill to using Twinway.
"LIVE FREE OR DIE...DEATH IS NOT THE WORST OF ALL EVILS." Gen. John Stark. "by reason of much foule weather and Extreme Bad Woods to travel in..." From the letter of my Great Uncle, Samuel Willard (accompanied by my grandfather Henry), to Governor Dummer on August 16, 1725, explaining the reason for his return, being instructed to "range all the country", of the Wawobadenik (White Mountains) July 19-August 16, 1725. I am a 13th generation New Englander and proud of it.