If I was you I would skip AMBBR and do Tour De Tahoe instead (on Sept 9th). I have done both so these are my experiences. AMBBR is a huge event, people from all across the US show up. Team In Training uses it as their fund raising event. What this means is there are thousands of cyclists on the roads of varying skills. This leads to a lot of crashes on this ride, and I saw a few downed riders. Last year someone had to be air lifted out. They force you to ride on a MUT (Multi Use Trail) which is rather dangerous in my opinion. You have people who think they're Lance Armstrong and are hammering on the trail. There are also various metal poles on the trail as it intersects with walk ways. The ride out to Truckie sucks and there was a strong head wind on the way back and not much to see. I think anyone who has done AMBBR before pretty much agrees that riding around the lake is the part only worth doing. The ride around the lake is 72 miles, they add on the ride out to Truckie to get the full 100 for AMBBR. When I did Tour De Tahoe, it was much more mellow and not as crazy as AMBBR. You don't have to ride on the MUT either. And they cut out the ride to Truckie.
I don't have any experience registering the day of but its a huge event so I am sure there will be some no shows and if you show up early enough you may get it in. But you can basically do this ride anytime by yourself. Just keep the lake to your right.
