If Beacon is like Syringa on steroids, than Riverside is like Beacon on kryptonite...or something like that. It is big. There is lots of riding. It is a relatively short drive. And best of all, it is rideable for much of the year.
Anywho, I'm sure most people have ridden there before. Yes, there are lots of views and the river running through it ain't bad either. But have you done the 25 Mile Perimeter Trail? No. Why? Oh, cause a section of the 25 Mile Trail goes through the equestrian section of the park and that is closed to mountain bikes. That's lame. Now, I would never condone riding in a closed area. Although I'm not quite sure how they intend you to cover 25 miles if not by mountain bike or horse or motorcycle. Oh, maybe it's hiking. That is lame too.
My point is, you can do most of this trail on your bike, legally, and avoid these closed areas. Get a map. If you do, you will be rewarded. You will see sections of the park that most never have. You will ride in areas and forget that you are even in Riverside anymore. You will see all kinds of terrain: sand dunes, plateaus, double-track, rocky descents, flowy single-track, rocky ascents, Vietnam like river plains..you get it.
Now go do it. My camera batteries died, but here are a couple pics...