Tomorrow, June 7, is National Trails Day. There are activities happening all over the U.S. and you can probably find one near you with a simple Google search. Here are a few links that I found of groups that are doing things like work parties, trail rides, hikes, etc.:
http://www.americantrails.org
http://www.americanhiking.org/national-trails-day/
http://www.backcountryhorse.com