It's actually closer to 12 miles for the complete circuit (all loops). I've done Fantasy five times now, and done a lot better each time. My first time I didn't keep exact time, but I know it took me around two hours including occasional breaks and a few close calls.
Now I'm a lot more comfortable there, and in better shape as well. I'm riding an '07 Rockhopper Comp and I just bought a pair of clipless pedals for it, which really make a considerable difference. I did Fantasy two weeks ago and timed myself to see how fast I could do it. I arrived back at the picnic table (entrance) after 58 minutes. I took no breaks (thanks to a Camelbak), but had one five minute stop after my friend bailed on the over-under.
I'm confident I can do it in 50-55 minutes the next time I go out there. Even at that pace though, I've been passed by several other riders, so I imagine some people can get it done in the 40 minute range. I've always wondered what the record is for Fantasy.
I've been heading out there near 5 am the past few times, just to get it done before it gets too hot. It's beautiful, except the sun starts getting in your eyes as it creeps over the Rincons. I actually think 6-7 would be a better starting time.