Pretty hard to go from Lexington to Washington, DC without crossing the Appalachian mountains! Interstates will have the straightest and least severe grades, but it still means going up and down. Also, both direct interstate routes take you through West Virginia, which might be 'Almost Heaven' but more hilly than Hell.
I-64 to Charleston, WV, then either S to I-81 N, then I-66 to I-270. Going so long in WV is pretty hilly, but the alternative, at Charleston WV go N on I-79 to I-68 to I-70, then I-270 to Washington also has it's share of hills and traffic.
If you want less hills, go N on I-75 to Cincinnati, then I-71 to Columbus, I-70 E, PA Turnpike, I-70 at Breezewood to I-270 to Washington.
If you pay attention, the WV hills won't hurt you. Downshift when climbing to prevent engine 'lugging' in too low a gear, Downshift going downhill to use engine compression braking going down. Only use brakes to drop speed by 10 mph or so, then let your foot off the brake pedal until next need to drop speed by 10 mph. Overheating brakes happen when you try to use the brake to control speed. Apply to drop speed by 10 mph, then let them cool.