You'd save at most 500', or a two minute walk at normal speed. The road isn't so much a problem: you can't walk across it to get to the garage except via the skywalks above the departures level -- and you'd lose more time going up/down the skywalks than you'd save by walking directly. Plus, the airport's rarely so crowded as to be unpleasant, and has a few amusing exhibits along the way.

If you really, really don't want to walk, the every-few-seconds BWI shuttle buses make the trip.

While you're waiting for the train (it's not that frequent), you can buy tickets, use the restroom, and peruse the maps inside the international terminal. If you have a WMATA SmarTrip card, you can use that on Baltimore transit, too.