East coast “downcountry” loop reccomendation

Sterling Foest State Park and Ringwood State Park are both about an hour from the George Washington Bridge and offer the most contiguous singletrack in the immediate area. https://www.palisadesmtb.org/ is the local advocacy group for Sterling and just opened a fantastic new trail this month. Our hills top out at about 1200' but there is still plenty climbing and descending. If your willing to drive 3 hours for more elevation there is some great but somewhat limited riding in the Catskills. Roundtop and Elm Ridge are fun and there is some even better stuff in the western Catskills that isn't being promoted to the public quite yet. Feel free to give me a DM, these have been my home trails for 25 years. 

Make a weekend trip to Vermont. Kingdom Trails might just be what you are looking for. 

Port jervis is a good time.

Not down country at all.  But mt creek is an absolute blast and a must stop if you are in nyc imo.

In the greater NYC area there are a lot of high mileage dudes riding all over the place. Sprain Ridge. Blue Mountain. Graham Hills. Just check trail forks for the blue trails that get ridden a ton and see the popular loops. 

Depends how much you want to travel. Ascutney, Waterbury, kingdom trails, and Stowe Vermont. Are all worth traveling to. As is Conway NH. 
 Western Massachusetts has Thunder mtn bike park in charlemont. Batchelor street and earls trails in Amherst/granby. Pittsfield state forest and Lenox. Ludlow. 

I'll second Port Jervis NY, lots of mileage and variation.

Port Jervis can be exactly "downcountry"..... There are about 5 pedaly "enduro-esque" trails and if you do them all in a day you're looking at about 3500ft of climbing without linking normal connections between them. (cosmic charlie, 5 bridges, flintlock, painted aprons, and Tomahawk). Sterling is a good option with lots of ungelating terrain, gets super hot and humid, and most of it is a loop so not much getting lost. Ringwood is a bit more just trail riding but good and rocky and techy, Glen Park in East Stroudsburg, Pa has held a ton of enduros and is alot of fun for riding laps and climbing to your hearts desire, and if you can find them there are some crazy good trails in the XC zone of Mountain Creek.

Normally I wouldn’t point out grammar/spelling errors and I am doing so here in good faith and with intended humor. Not trying to be a dick in any way. 😘

But spelling undulating as ‘ungelating’ is diabolical. It reads like a problem where one can’t get their jello to set properly. Haha

Camp Alpine on the Jersey side of the Palisades has the goods you are looking for. Would also second Ringwood/Ramapo; there is some brutally techy stuff in there. Jungle habitat is a bit smaller, but an interesting setting, and again, brutal tech. Chimney Rock in boundbrook can be linked together for some really nice ups and downs over big miles, especially the new Vossler side. The mountain creek XC trails are above and beyond, but I don't recommend going without a local first, as they can be very confusing, and there are some large features back there. Out of all of these and those mentioned above, Port Jervis will give the most "Downcountry" feeling, lots of good climbing trails with descents that are worth the effort.

I would definitely second the Blue Mountain recommendation.  When I used to work at a local shop in the area we had lots of customers come up from the city to go riding... You could even take The Hudson River line there.  The one place I would put above that for what you're looking for would be Mount Beacon.  Very easy to bag 1,000 to 1500 ft in a single climb.  You can easily find 10 to 15 mile loops and quite a bit more if you're feeling sporty and head south towards Cold Spring.  This place is also accessible from the Hudson River line and is 100% worth the trip.

..... There may be a nice, cool large body of water at the top of the fire road on Mount Beacon, just give the area near the dam a little bit of room because of the intake current.🤙

....or if you REALLY want big loop you could travel up to the ADK and do this....

https://www.trailforks.com/route/over-easy-starting-at-blueberry-and-ending-at-otis/

...plenty of swimming holes on this route as well!

As someone who lives within a short bike ride of Camp Alpine, I can assure you those trails aren't what you're looking for. If I had to drive there, I’d skip them altogether. They can be fun local loops, but they’re unsanctioned legacy social trails with nothing special in terms of terrain or mileage.

@longroadtonowhere I'm not sure what your exact "downcountry" setup is, but almost all the suggestions here, aside from Mount Beacon, Mountain Creek, and Glen Park, are doable and fun on a 120mm bike. Those three really warrant at least 140mm of travel to enjoy. Also, be ready for brutal heat and humidity, we're expecting triple digit heat indexes later this week, and the air in the woods will be still, heavy, and full of bugs. Here are two Ringwood routes to help you get the lay of the land, https://www.trailforks.com/route/rowdy-ringwood/?activitytype=1&z=12.5&lat=41.12760&lon=-74.23211&content=trails,labels,poi,directory,polygon,waypoint,nst,routes_featured and https://www.trailforks.com/route/ringwood-big-loop/. Sterling is stacked loops and relatively easy to navigate, everything there rides well in both directions.  

Shandaken Wild Forest out of the Lost Clove trailhead is now up on Trailforks. That was the network I was initially hesitant to share, but since you're looking for Downieville training, it's the only local network that comes close to offering a comparable experience. You'll find two uninterrupted, high-speed, 3 mile descents that are perfect for a short travel trail bike. Unfortunately, we just don't have the 10 to 15-mile sustained descents you find in the Lost Sierras or elsewhere out West, but running laps here will definitely help build the descending fitness you need for Downieville.