Slitherine

Does the distance to a supply source affect anything?

it doesnt

Anticipating a future question: What does happen, though, is that players forget the zone of control expansion feature. That is, they will open a supply route which at one point or another skirts around an enemy unit. This could be even a bunker or foxhole. As soon as the player moves his units away, the enemy unit expands its ZOC by one hex all around and possibly cuts off a supply route that is only one hex wide. Rule: Either make sure that a supply route is cleared of enemies on both sides by at least one hex (and remains that way, of course, in the event those enemies are mobile) or make the supply route three hexes wide from start to finish.