Cathy, glad you asked! Below is a list, in roughly west to east order. Some of these trails are big and well known, some are much shorter and less known, but they're all just waiting for you to explore.
All the websites follow the same design pattern with the interactive map, trip planner, timeline, etc - although some of the smaller or newer ones do not have a forum, since they have less traffic.
And the list is always growing.... Before the end of the year, the D&R Towpath (east/central New Jersey - New Brunswick area - 77 miles) will be on the TrailHub website, where you can also find the full list below: https://trailhub.com/
Beyond that, I'd like to eventually add Coeur d'Alene, Ohio to Erie, and whatever other good suggestions/ideas may come along.
South Dakota -
Mickelson Trail
Nebraska -
Cowboy Trail
Texas -
Northeast Texas Trail (from outside of Dallas to almost Texarkana)
Minnesota -
Paul Bunyan Trail
Missouri -
Frisco Highline Trail (southwestern MO)
Missouri -
Katy and Rock Island Trails
Wisconsin -
Elroy-Sparta Trail
Illinois -
Hennepin Canal (from near the I&M Canal to quad cities/Iowa state line)
Illinois -
I&M Canal (from outside Chicago to near the Hennepin Canal)
Illinois -
Great River Road (Alton to Grafton, north of St Louis)
Mississippi/Alabama/Tennessee -
Natchez Trace (from southern MS to Nashville TN)
Alabama/Georgia -
Chief Ladiga (AL) and Silver Comet (GA) Trails
Michigan -
Border to Border Trail (Ann Arbor area)
Ohio -
Little Miami Scenic Trail
Virginia -
Virginia Creeper Trail
Pennsylvania -
Great Allegheny Passage (GAP)
Maryland/DC -
C&O Canal Towpath
New York -
Erie Canal
New York -
Empire State Trail
Connecticut -
Farmington Canal Trail (New Haven to Massachusetts - south/north)
Connecticut -
Air Line Trail (outside Hartford to Massachusetts - west/east)