I did this last year,
I left Cucamonga on Friday night. Got stuck in a 3 hour traffic jam at the CA/NV line. I realized I had forgetton my shoes. Got to the hotel at 1:30AM. Debbie(my roomate) Talked me in to doing the metric. It went out to Red Rock canyon. Very pretty, but I was pretty misarable. We got to the second rest stop, which was in front of a LBS at Blue Diamond. I got out my plastic and bought a new pair of cycling shoes and cleats(The tennies I was wearing had just about had it!). It was windy as all get out going back.
I'm planning on doing this, dependning on my shoulder. The 100 miles looks like it does both Red Rock and Hoover Dam. They have a metric and a 50 , too.
Cali - Tom/Sue, Fred/Sylvia, and the rest of The Cycling Connection crazies should be there. It should be fun. It's also low key. I think I remember they only had 800 riders last year.