Distance: Any length you want! A 5K gets you to Chelsea; a 5-miler puts you in the Lincoln Center Area (from there, you can easily walk/run east to Central Park); a 10K and you can unwind and relax at the beautiful 79th Street Boat Basin; an 8-miler and you can stretch in Riverside Park; and a 12-miler puts you on the George Washington Bridge.
Best time to go: Aim for early morning or early evening — you'll be on a running/walking/biking path that gets very congested throughout the day.
Route: For most of the run, you will be running alongside the West Side Highway (West Street or 9A). Just head north. If you plan to head up to the GW Bridge, you will end up running east for a bit (around 158th street) to access the ramp up to the bridge).