This is my first post in this forum :)
I did the trip to Tehipite Valley a few years ago. The switchbacks are exhausting for sure. Also, we encountered 2 rattlesnakes on the way down and then the last third of the trail was covered in leaves and some gravel which also made it slippery. The trip back up the switchbacks was actually a lot easier.
There's a book, a true story, that centers on what happens down the switchbacks to Tehipite Valley. It's called "Angels in the Wilderness". I haven't read it but saw it at REI. Anyway, I'm sure your trip will be incident-free, unlike that of the author :)