This would be a very nice hike under most circumstances, linking together many of my favorite trails on and around Mt. Wilson... unless you try to do the whole thing in one epic day! But I designed it carefully so that exactly that is possible. I believe it is about 45 miles, with at least 8300 feet of total elevation gain. (Robinson doesn't give mileages for all the legs, so it could be plus or minus a mile.) There are convenient water sources approximately every 10 miles, and most of the trails are well maintained and easy to follow.
The route starts and ends at Chantry Flats. The first leg follows some of the most beautiful trails in the area. It goes up to Spruce Camp then up to Newcomb Pass east of Mt. Wilson where it follows the gorgeous Rim Trail up to Mt. Wilson. There's a water fountain here by the huge parking area, which you are likely to have all to yourself if you get a sufficiently early start and are making good time. This is about 9 miles, +3700 feet, -200 feet.
The second leg descends the steep Rattlesnake Trail down the wild back side of Mt. Wilson to West Fork camp. Here it joins an unexciting fire road and climbs up to Red Box Gap. There is another water fountain here. This is about 10 miles, +1500 feet, -2500 feet.
The third leg continues past Red Box down another unexciting trail to the Switzer picnic area, one of the most beautiful but popular destinations in the mountains. From here it heads downstream, descends to the bottom of the impressive falls, and continues downstream to Bear Creek. You should be able to find one or two reliable springs along lower Bear Creek. If you don't trust spring water you should carry a chlorine / iodine tablet to treat the water. This is a wonderful secluded spot that few people visit; it is a good place to take a short mid-day rest. This leg is only about 6 miles, -2100 feet.
The forth leg heads up rugged pristine Bear Creek. The trail is indistinct in this section, so keep your eyes open for the route climbing out of the canyon on the right (south) side. Follow this dry hot trail up to Sloan Saddle, then up to the fire road above, which will take you past Mt. Markham and San Gabriel Peak (and through the cool tunnel on the precipitous south slope of that peak). When you hit asphalt you start the least pleasant stretch of this trip: follow the main Mt. Wilson road back up to the big parking area where the first leg ended. This is at least 12 miles, +3100 feet.
Get your last water at the fountain here, then start the long painful descent back to Chantry Flats via the beautiful toll road and Winter Creek trail. This descent is the final forth leg. Take the Upper Winter Creek cut-off trail a couple miles from the end that contours high above the creek instead of following the creek down to the Sturtevant trail. It would be wise to be familiar with the forth leg before trying the whole loop in a single day, because you are likely to do at least part of it in the dark. This final leg is only 7.5 miles, -3500 feet.
When I did it, all I carried was four Bear Valley Pemmican Bars. That's it! It's a very cool feeling to strike out into the wilderness with no pack or water. But since you are traveling so light (take running shoes or the like, as well) you can make fantastic time. I made it to the top of Mt. Wilson before it even opened to the public -- that's usually a hard half-day hike. Heheh, and you're just getting started! :) I was feeling pretty bad by time I reached Eaton Saddle and downright miserable on the knee-killing descent down Upper Winter Creek trail. I did the last few miles at a jog trying in vain to make it back to my bicycle at the trailhead before dark. Ugh, and then I had to ride 10 miles home in the dark! I had tendinitis the next day and could barely walk for a week -- the things we do "for fun"...
(An interesting alternate route: after Sloan Saddle, turn right on the fire road above instead of left, and descend to Idlehour, then climb up to Mt. Harvard via the old toll road and rejoin the other route at the top of Winter Creek trail. I opted for the other route because I don't trust the water in Eaton Canyon at Idlehour. (And I happen to think the tunnel on the face of San Gabriel Peak is totally awesome! :)