It seems like something is broke with distance mates in 2015. In all previous versions when adding a mate that could be coincident, parallel or at a distance it would default to coincident. You could then change that to a distance mate & it would start out at the distance the part currently is at. It still does this in 2015 but it seems like more often than not when you change the distance to what you actually want, Solidworks throws up an error that this mate will overdefine the mates even if it's the only mate that's placed. You could also take a coincident mate that's not overdefining & change that to a distance mate which then either throws the overdefined flag or Solidworks tells you that this mate can't be added or changed because it will overdefine the part.