Hi tom_c. Haven't I seen your posts on another board somewhere? Anyway, welcome!
The Fuller Ridge trail is a real good choice, although the drive to the trailhead can take quite a while. Once you're at the trailhead, it starts out kind of mild. Then you pass a sign that says you must have a wilderness permit, You would have to turn around and drive back down to get one, so remember to get one in advance so it's not a problem.
The trail pops out at what I believe is Castle Rock, about 1/3 of the way to the top. You get a great view directly across a huge canyon, looking directly at San J. Great photo op, nice place to stop for a snack. When you resume your hike, you'll find yourself (oddly) going down very steeply. You'll have a couple of miles of ups and downs, and it always strikes me as odd to climb a mountain by going up and down, but that's how the trail is engineered, so there you go.
You'll probably cross a weakly flowing creek when you turn left onto the Marion Mountain trail heading up to Little Round Valley. Not tons of flow, but I drank from it freely a couple of weeks ago, and it was fine. There's water (or was two weeks ago) in LRV.
All in all, a very nice trail, and the weather at the end of September is generally very close to ideal.
To watch the forecast, I'd recommend this link:
http://forecast.weather.gov/MapClick.ph ... -116.67625
Hope you have some fun out there.
z