I'm looking to start walking, adding distance each day, and because part of the walk would include a train ride to get into the city (walking is more interesting in NYC) I prefer to use an iPad, so I can read or watch stuff during the hour+ train ride each day. How accurate is the iPad as a distance tracker or pedometer? The price of an app is not a concern, but accuracy is. While walking I'd keep it in a backpack, so GPS would probably be best, right? Price is not an option, and it can disable while on the train while I use the IPad for other applications. Any help from those "in the know" would be GREATLY appreciated!