I ran cow mountain last year and after a weird crash that pulled the cap off my radiator and spilled all my water plus various other misadventures, houred out around the 70mi mark.
It was wet and sloppy! I can't say how hard it would be on a dry course, but I was impressed with a couple guys motoring right along on xr400s. I wouldn't want to have tried it on an xr650, but best of luck!
There are some videos on youtube, check out the first "special section", that was the most technically challenging as I recall. There were a few other areas that were fresh cut through the mazinitas that were challenging as well. I ran into a couple bottlenecks after I got behind, but I didn't actually find them very hard to get stuck, you just needed plenty of momentum.
Cow mountain is also a restart format enduro and there is no time keeping, so you just go as fast as you can for the special sections and don't need to keep to any averages.