Here are some suggestions, all in the San Gabriel Valley but not too far from the SFV:
1) Cherry Canyon in La Canada. There are three main trailheads, in a residential neighborhood, and a variety of connecting trails including fire roads and single tracks. There's a radio tower from which there are great views of Los Angeles out to the ocean on one side, and the San Gabriel mountains on the other. Uncrowded, seems like mostly La Canada locals walking their dogs, etc. There is some mountain bike traffic.
2) Echo Mountain in Altadena. This is kind of a classic hike, about 5 and a half miles round trip, with a steady, switchbacking ascent. Not much shade so it's a good morning option. Can be very crowded especially on weekend mornings. At the top there are ruins from an old mountain resort which are pretty cool to explore. For a beginning this would be kind of a tough hike but it's doable. Do this one a few times and note the improvements in your fitness.
3) Jones Peak in Sierra Madre. Trailhead is at Bailey Canyon. This one is simlar to Echo Mountain in terms of length and steepness, but it's less crowded.
