I went out to Riverside after work, and did most of the 24-hour course. As you'd expect, there are big standing puddles on the forest doubletracks. I wouldn't call it muddy in the proper sense; you and your bike will just get sprayed with lots of silty water, as opposed to "frosted" with caked-on mud. For the near future, expect to get fairly wet.
On my way to the Start/Finish area, I looked down into Little Vietnam and saw that the singletrack by the river has extensive standing water. I also saw one of the Rangers' signs saying that the area's closed until the water goes down. So I suggest bypassing Little Vietnam for now, whether you see a Trail Closed sign or not (evidently people are tearing them down)... just cruise downriver on the Centennial Trail and jump onto the inner loop from there.