What you drive, where you drive it, how far you drive... none of it has any effect. If you complete your daily workout/marathon - which is 26.2 miles, or 42.1 kilometres, as the distance of a marathon - you get the next car on the list. It might be new, it might not. You can't game it or influence it in any way.
It simply appears that there's a giant list of car ID numbers (and GT5's used car database list was 2.167 billion days long, so this could be a similar magnitude) and you go from one to the next each time you hit your daily workout. I've always suspected that the list doesn't directly correspond to valid car IDs, but simply rolls over until it finds one (that way the list doesn't need to update each time a new car is added), but this paragraph is largely unconfirmed and pretty much just reasoning based on observation.