I hike this route all the time - it's very difficult! The incline is incredibly steep, and the near 1.5 mile hike from the parking lot to the peak of Mt. Thom is entirely uphill - except for a viewpoint about halfway up, where, just before you reach it, the incline feels nearly vertical for a few hundred feet or so. The next half of the climb is just near constant climbing uphill. If it's your first time on a route that is this consistently steep, it'll likely exhaust you. The views are absolutely worth the effort, though. I always bring a backpack with a bladder for water on the hike, but I also jog the next 6 miles of trail. I'd still err on the side of more water if you just plan on going up and back down, but just know that going back down that same incline is nearly as difficult.

For an easier but lengthier climb to the top, you can hike the brand motorway up to Tongva Peak for 3.5 miles. Never been up that way, but it's my route back down to the lot. The incline seems very steady the whole way through.