How do I make a distance traveled Maps (yahoo/google) feature

I’ve been asked to make a feature that works with one of the popular map APIs. This feature would take a Start Point, End Point and a distance value and would generate a map showing the progress from the Start Point to the End Point.

For example, if the Start Point was Seattle, the End Point was Chicago and the distance value was 1000 miles, the map API would be used to create a map showing both Seattle and Chicago, with a line about 1/3rd of the way there.

Ideally, the mapping API used would be a publically available API available without cost. Seems easy, right? I’m not sure where to start. I would appreciate all input at this stage.