The Grand Staircase in Fort Tryon Park leads to the Met Cloisters. A set of long park stairs is the perfect place to get a serious workout.   And if you live uptown, you're never too far from a hilly park crisscrossed with an impressive staircase—many have more than 100 steps.   So here's an idea: Find the nearest and get ready for a major sweat session. A good rule of thumb is to run up and down for 30 to 45 minutes. (Of course you can walk instead.)   From south to north, h