Is there anywhere in the northeast where one can do a really long (like 10-20+ miles) of only 1-2% grade?

I'm not sure about that, but you can find plenty of 12+ % grades in the Lehigh Valley.

At only 1-2% grade, ride into a headwind and shift into a smaller cog.

Try finding a road along a river valley near the Appalacian Trail. Or going into a mountain range. In Colorado the road between Buena Vista and Leadville would suit you, aside from the high altitude. I checked the roads paralleling the Delaware river and they were way too flat.

Yeah, I thought maybe there might be a 'top of mountain ridge' road that might have those characteristics. Lancaster Ave near Philadelphia almost has that characteristic, going from zero to about 500 elevation in about 19 miles but with a few dips. And it is not a bike-friendly route.

The North Central Railroad Trail, which is about 20 miles from Ashland, MD to the border of PA, is about 1-2% for the northbound trip. Once you hit the PA border, you can continue on all the way to York, PA, on the York County Heritage Trail, which is about another 20 miles. I've only ever ridden a few miles on the PA side; it was still slightly uphill at that point, but I don't know how it is further along.

It's a dirt trail, but on the Maryland side, at least, it's fairly hard packed. I usually ride it on my 'cross bike with size 32 tires at 75 psi. I've ridden short stretches with the 25s at 100 psi, and that's been fine, but I'm not sure I'd want to ride 40 miles on them.

https://www.bikewashington.org/trails/ncr/ncr.htm

I rode the York County Heritage trail last year and it has a nice slow gradual uphill grade heading north. Not too far past the PA line it starts a very slight downhill to York. On the trip south, the grade from York to New Freedon (at the PA.MD border) is barely noticeable. From New Freedom to the end has a slight but noticeable downhill. It is a very nice riding trail on anything but really skinny tires. It is also the most amazingly clean trail I have ever seen. You almost think someone comes out and vacuums!!

That North Central Railroad Trail / York trail combo looks nice. I just typically don't do long linear trips, because either you have to go back on the same route or have someone drop off & move the car to the other end. Although I'm sure there's a good way to make this a century with 40 miles of path and 60 miles of way-too-hilly terrain.

Campton NH to Waterville Valley NH. 12 miles, I believe.

Check into the Erie canal trail between Lockport and Rochester NY. That is like 50 some miles and an elevation difference of only a couple of feet.

The Great Allegheny Passage also fits the bill. I ride a section of it from Ohiopyle to Rockwood which is 30 miles one way. That ride back is always a lot faster, even on crushed limestone.

Denny

Skyline Drive: Front Royal to Thornton Gap


Has anyone else done this route?

It might get a little steeper in places, but the North County Trailway in Westchester, NY might fit the bill. You can go from Eastview NY to Carmel NY, about 30 or so miles one way, 60 miles round trip. Mostly rail trail, though you ride Rt 100 for a couple of stretches.