How long is 'long?'
In my home state of NY:
NY Long Path is a mixed bag. Official distance for a thru is just under 350 miles, NYC to Altamont. The start of it is "urban greenbelt", and Orange County is a problem (even the sponsoring organization recommends that thru-hikers detour on the AT and Shawangunk Ridge Trail), but the Shawangunk Ridge and Catskill sections are awesome trail. Riggsville to Conesville is about 120 miles and highly recommended. Some of the best hiking that the Catskills have to offer, and Phoenicia is a great little trail town. Onward from there to Thacher Park is good if you don't mind it being about half roadwalk. Enough is in the woods that there's no problem finding a place to camp every night. (I haven't done nearly the whole thing, but I've done almost all of the Catskill and Shawangunk segments in dribs and drabs.) Beyond Altamont, it's a blazed roadwalk with a handful of short off-road jaunts as far as Northville, and the rest (to Heart Lake, Lake Placid, Whiteface or Jay, depending on which description you read) is definitely a route, not a trail. In retirement, I want to do the whole thing, Manhattan to the Adirondacks, just because I think hiking from Manhattan to the Adirondacks is a cool idea - a real ritual of 'leave the city behind.' It's too big a project to do while I'm still a working stiff.
Northville-Placid is now 137 miles. It used to be 125 - the way the article said - because the Northville roadwalk was optional, but another big section got moved off-road. It's low-elevation so the views aren't great, and it is wet most of the year, but it's really pretty, and very remote. There are two towns that are perfectly positioned for resupply (although Piseco is a maildrop now, the general store there closed). Recommended that anyone who does this one be comfortable with at least short bushwhacks, because there will be unblazed detours around closed sections. The beavers don't announce their construction projects in time for the trail maintainers to construct reroutes to accommodate them. I had fun thru-hiking it; it should be on everyone's "Thru Hikes You Don't Have To Quit Your Job For" list - a real thru-hike in miniature.
There are some nice sections on the Finger Lakes Trail, and some crappy ones - like any long trail other than the Big Names, it's never quite finished.
The trails in the Adirondacks all connect, so you can construct various routes. One that's starting to catch on a tiny bit is the 240-mile Trans Adirondack Route - definitely a route rather than a trail, travel on logging roads and bushwhacking required. Only thirteen hikers have managed to thru-hike it so far. The web site for the trail documents nine parties' failed attempts as well. My opinion, just from the route description, is that the designer had to include some 'substandard miles' in order to make the route run from Blue Line to Blue Line. If I were to do a hike of that length in the 'Daks, I'd try to explore Vince Schaefer's route from Northville to Heart Lake, then loop back on the Northville-Placid to the starting point. Route-finding would likely be challenging northbound, since all the descriptions are from the 1930's. It's at least possible to do it lawfully again, now that the state bought out Finch Pruyn and you can hike Boreas Ponds legally.
The North Country Trail in the Adirondacks doesn't really exist yet. The route is sort of chosen, but there are sections where it's not yet even been flagged. (That said, it's been hiked. The ADK's have some crazy bushwhackos.)