I have hiked Dawns Mine which is rated difficult at 6 miles and loved it. I want to do "bridge to nowhere" its rated moderate but is 10 miles, does anyone know what the rate is based on?

I don't. But Ive don't it many times. Two things: 1 stick to the trail on the right side of the canyon as much as you humanly can. 2 it's not strenuous activity but most people's feet cannot handle the uneven terrain. If you don't pay attention to #1, #2 gets way worse because you walk on river stones the whole time.

As Trevor said stick to the trail on the right, you'll still have to cross the stream of the San Gabriel River a few times, the hike is mild to moderate. If it's hot take enough water or a filter it's a fun 10 miles round trip hike.