Although it sounds daunting, biking from the northern tip of Manhattan to the southern tip is easy, fast, and free (if you own your own bike). Riverside Park, which stretches nearly the entire length of Manhattan along the Hudson River, provides bikers (along with walkers and rollerbladers) a unique view of the city, the river, and New Jersey.
The entrance to the park is on Riverside Drive where it meets the river. Follow the small signs for bikers and you'll be sure to find it. The first one to two miles is actually the worst and hilly. It is a gradual incline from the start until the 181st Street entrance. Although it doesn't look like a bad hill, for out-of-shape bikers, it may be tiring. The path then twists and turns downhill, which I think is the nicest part of the trail. However, you must be careful to go slow and look out for people.
The next couple miles prove to be the most congested (on weekends, especially). People from the neighboring streets usually flock to the river for picnics, which can be quite a hassle for serious bikers. The path can be overcrowded with walkers and little kids, and they don't really care if you're on a bike or not. So, at this point, be prepared to stop often. Some people like to bring small mopeds on the trail at this point too, so be careful.
Once you break out of this area, the path then goes underneath the West Side Highway, near 125th Street. Follow the signs for the bike path and you won't stray.
Once back in the real park, the path is wedged between the West Side Highway and the Hudson River. The path is divided in half for foot traffic and bikers. The path continues right next to the river for several miles before reaching 96th Street, which is usually when it becomes quite congested again. There is a point near here where the path is blocked by large rocks. You can go over or around the blocks, but the path becomes extremely narrow and rough. As an alternative, follow the signs and take the path that parallels it, further from the river near the street.
Below 59th Street the park becomes all concrete. . . no more grass and trees. Lots of foot traffic from midtown also makes its way to the path, so be careful.
As you approach 34th Street and 14th Street, the path becomes separated into two paths. . . one for foot traffic, one for wheeled traffic. Look out for strays, though. The path continues in this fashion for the remainder of the ride. Along the way, you'll pass the Intrepid, Chelsea Piers, and eventually Ground Zero. The path ends at the entrance to Battery Park.
This ride only takes about 75 minutes to complete. For a more leisurely ride, give yourself two hours. If you're in shape, you can always ride back to Inwood on the path. Otherwise, do what I do. . . take the subway back. The A train is on Chambers Street.