Steve, our guide, was very encouraging and knew the terrain well. This was my first time mountain biking. We were taken to Brown's Ranch Trailhead and rode 7.5 miles, 525' elevation. It wasn't easy, but it was beautiful. We had a nice small group, 5 people. The guide provided snacks and water too! Additionally, he provided info on history of the land and people as well as some tidbits about the environment. There are a lot of trails out there and we ended up going on parts of several different loops. We saw Cathedral Rock and other interesting features.
For other newbies: We all rented our bikes from REI. Half went with the upgraded models and said they really liked them. I had the basic bike which seemed fine for this trail. I had a little difficulty setting the gears comfortably and was constantly aware that I did not want to crash into a cactus. When leaning against the bike, beware of the spiked pedals and the chain. As a newby, I brushed against them and got some grease and small cuts on my leg - NOT a big deal. And I never crashed (into a cactus/chollo)!