Jackson Heights is about as flat as they come. With no changes in elevation, the neighborhood makes for a poor location for hiking or any real extensive exercise. However, with its grid layout and urban feel, you can certainly look forward to taking long walks with plenty of things to see and do while you're out for the day. However, you'll want to be a little bit cautious, as New York weather in the summer can mean that the pavement can get too hot for sensitive paw pads.
Thankfully, Jackson Heights' artificial terrain can be broken up here and there by some of the smaller green spaces that act as small playgrounds or tiny fields which are good for a quick break from the hard asphalt and concrete. Just be sure to check the local rules and regulations, as you may need to keep your dog on his leash while visiting.