Dan Campion just sent this to me and I think it would be a lot of fun for anyone in the area who is interested. The ride from Siren to Webster is about 6.5 miles for a round trip of around 13. If you're really feeling ambitious, I suggest you leave St. Croix Falls at around 9 and intercept the group on the trail (you'd end up with an 80 or so mile day)!
This should be a fun/casual ride, so if you're looking for something to do to celebrate national trails day, this is a good option! Plus, the Gandy is free on that day, so what do you have to lose?