Great for beginners who would like to try out some challenging technical sections.
Signage suggests that the trail may cross into private property at any given moment. So long as you stay in the arroyo you shouldn't have any problems.
The St. John's Arroyo Trail seems to have been born out of a fair amount of use over time. There are no maps and the trail often splits off in a number of directions. While most people start at the St. John's parking lot, the trail actually begins (or ends) at the dogleg turn on Old Santa Fe Trail (west of St. John's College).
This short 1/2 mile section crosses the arroyo a number of times and can be a bit confusing as there are splinter trails and the expectation that you just ride in the arroyo for stretches. It connects with the St. John's Trail just east of the underpass and from there you can head up Atalaya Mountain or you can continue on in a southeast direction along the arroyo (if you are headed out).
From here, you follow the arroyo for a while before finding the singletrack again. Once you find the singletrack, you'll ride a slight incline for roughly two miles before the trail comes to a makeshift gate. The trail seems to end here as you'll travel on a jeep road if you choose to go further.
This trail is often ridden as a conclusion to the larger Sierra Pelada Loop. All of these trails are unmaintained however and you could easily get lost. Returning on the trail is a blast, as it is slightly downhill most of the way.
Overall, the ride is fairly easy with some tricky, technical sections sprinkled in. It's great for those that are new to mountain biking. It's also popular with hikers and dog walkers so be careful and give them the right of way.