Other Nick, you are terchnically correct, but we are talking about tourists here. If you tell a tourist they are one block apart, they will expect Michigan to be where Wabash Street is located. From the perspective of a tourist, it is two blocks. Sometimes giving applicable directions is better than being technically correct.
Like the others were saying, the stores are on State just North of Monroe and South of Madison. Michigan Avenue will be two large streets over (so don't turn on Garland, which is basically an alley). Once you get to Michigan, most of the popular shopping will be North of the river. It is probably a six to eight minute walk to the river once you get on Michigan, and it will probably be another ten minutes to Water Tower Place. All-in-all, figure about a twelve minute walk to the start of the Mag Mile and another ten to the North side of the Mag Mile.
If you are talking about the Metra when you ask about the train, the last stop is probably the closest. This will be Ogilvie, Union Station, or LaSalle Street depending on the line. If you are talking about the El (subway), there are a variety of stops nearby. Monroe on the Red Line or Madison/Wabash on the Loop (many lines) would be closest. If you are considering taking the train to the Mag Mile, don't. It is more time consuming than it is worth.