Or 39.9 if you're splitting hairs. The last time I did a ride this long, I was at UCSC and it was the two rides from San Francisco to Santa Cruz. One of the things that I love about doing a decently long ride is that you get to see a variety of different environments. I rode urban streets, a fair amount of shoreline roadway, some random bike paths passing through areas without roads, rolling hills, and the world's second longest floating bridge.
It was also fun to do some real mileage with friends. We didn't talk a whole lot, but it's nice to share the ride and sprinkle in some drafting now and then. Of course it didn't hurt to be doing the ride with a guide - the whole loop is signed, more-or-less, but the more-or-less and transitions between road and path could have been difficult to figure out if I was on my own.
Around mile 20, my knee started to bother me a little bit, but we stopped and stretched and it calmed down. It feels pretty good at the moment. I think it helps to be riding with friends - without an impressive degree of organization it's a somewhat slower pace than when I'm riding solo. I was also trying for an 18mph pace as the point where I started to really push, rather than the 20mph pace I've been trying for on my shorter rides in New York.