I parked and headed East on the paved bike trail. The paved bike trail is part of the Santa Ana River Wilderness Trail. It runs from near Mt. Rubidouix west to some point in Norco. I've only explored a section of "horse trail" East of Van Buren.
Approx. 1 mile from the parking area is a trail leading through some high bushes. The trail was wet the day I rode through, however it was pretty sandy. So a ride through on a dry day, may prove to be more of a challenge. There are no technical sections to speak about, but the trail becomes interesting as you ride through tall thickets of bamboo and other tall "grasses".
I did come across a Riv. County Park Service sign that had warnings of rattlers (duh), mountain lions (humh), poison oak (saw some), and California black bears (HUH?!?!). I confirmed with a park employee that bears have been seen in the Santa Ana River Area, but very rarely...
The total ride was just over 8 miles. However, it began to rain heavily so I turned back.
I understand there are more trails (many miles worth) West of Van Buren to be explored in this area. However, watch for horses the closer you get to Norco is "word on the street.
From Van Buren & Arlington, head North on Van Buren. Turn Rt. on Jurupa. Cross over the rail road tracks. Park entrance is on the right side. Entrance fee is $4.00. (May be worth it if you like to explore and don't mind some "paved bike path" for a bit.
http://www.singletracks.com/item.php?c=1&i=1613