This is where statistics can really blind you, a majority of that is out west. Major population centers are on the coast. So you would still need to burn jet fuel to get there in a reasonable time frame.
The majority may be there, but there are still many scattered throughout the country.
FFS, there's a National Park in New York City: Gateway National Recreation Area. At least I hope it's still there: used to go mountain biking there a long time ago.