Yeah, from Heninger it's pretty much a fireroad the whole way... the two primary routes from the south are Little Santa Anita Canyon from Sierra Madre, and Big Santa Anita Canyon from Chantry Flats, and there are multiple variations of these.
Little Santa Anita (aka the Old Mt Wilson Trail, 15+ miles total) is my personal favorite (partially because I live less than 1/4 of a mile from the trailhead). It starts out a Mt Wilson Trail Park (Mira Monte Ave, about a block east of Baldwin). Hike up the road from the park to the actual trailhead, then you're on a wide trail with no shade for a bit (horrible in the summer). You'll reach the signed junction at First Water (reliable water source year-round, filter it), and the trail will start intermittently winding around north slopes that provide some shade. Eventually the forest grows more and more dense, and after 3 1/2 miles or so you'll descend a bit and pass Decker Spring, then quickly you'll reach Orchard Camp (again, reliable water source year-round, and probably your last until the top). Orchard Camp is roughly the halfway point.
From there, you'll climb back up through more of the same on the south flanks of Mt Harvard, with oak giving way to pine, and eventually you'll top out on Manzanita Ridge (signed junction with a bench, where the Winter Creek Trail from Big SA Cyn emerges as well). Head north and you'll quickly intersect the Mt Wilson Toll Rd. Follow the road up around Harvard, and at the saddle keep an eye on the right side to hop back on single track trail. Some nice views open up to the west of Lowe/Markham/San Gabriel Peaks and after a few switchbacks (~1 mile or so), you'll top out at the parking lot. The summit is huge, hard to tell where the high point is, but views are excellent and you can easily kill an hour or so just wandering around the observatory grounds. (snack bar is closed during winter, I believe)
Overall, it's a great workout with a pretty steep overall grade.
Big Santa Anita Canyon from Chantry Flats is a much more shaded hike, and there are also many more variations you can take... Drive all the way up Santa Anita Ave until it ends at Chantry Flats (parking can be a nightmare on weekends). Depending on which route you choose, this option tends to be a bit shorter (12+ miles or so) and with less overall gain, since you start at a higher elevation. I can't go into nearly as much detail, because the options are endless, but a good map will show you the way (
http://tchester.org/sgm/trails/chantry_flat.html). I'd recommend the route that heads up past Sturtevant Falls (Gabrielino Trail). Plenty of shade and water virtually the entire way, and past the falls you won't encounter too many other hikers. You'll also pass through Spruce Grove and Sturtevant Camps, and can opt for a different route on the way down to create a large loop. Don't go this way without a map (not because the trails are hard to follow, but there are approx 492 intersections).
All of the trails to Wilson are well traveled, maintained and easy to follow.
Another fun peakbaggery variety involves starting on the Little SA Cyn trail, leaving the trail and climbing Jones Peak via the steep south ridge, then cross the ridge to Hastings Peak, scramble up to the Toll Rd and make your way over Yale Peak, back down to the Toll Rd then up to Wilson... Try the normal ones first, though
