Michigan Department of Natural Resources:
DNR - Horseback Riding

Trail Riding Groups in Michigan:
Michigan Horse Trail Groups

Trail Riding:
Back Country Horsemen of America
50 State Trail Riding Project
Equestrian Outreach Riding Information
Tread Lightly
Equitrekking Trail Riding Quiz
Equine Land Conservation Resource
American Trails Organization
National Trails Training Partnership website resources
National Trails Training Partnership website on Trail Design and Construction
Trails/ Much information on trails, camping, gear, maps, etc. - Emphasis on hiking but the information is good.
3 Knots to Tie Horses

Michigan Trails:
Michigan Department of Natural Resources (DNR)
DNR - Michigan Parks and Trails

Michigan Endurance and Competitive Trail Riding:
Great Lakes Distance Riding Association   They have novice and beginner rides of 25 miles and less.

Other State Trails:
Tips for trail riding the Illinois Shawnee National Forest/    Southern Illinois
Trail Riding Indiana Horse Trails in the Hoosier National Forest/
Brown County Indiana Trail Riding
Hoosier Horse Camp
West Virginia horseback-riding - from Shalimar Farm/
Horseback riding in Maine - Kennebunkport, Popham Beach and beyond
Oregon Trails
Midwest Trail Ride
North Carolina
&New Hampshire
North Carolina
New York
USA Trail Ride Organization
Ohio
California
Kentucky

Other Trail Info:
10 Longest Hiking Trails in the US