The average trip on US (per pax, not per plane) is about 600 miles. Transcon for US is considered long haul.
Also, I assume you are talking about the press release. It doesn't actually say that the 321 will do long haul. It just says they will work along side the 757 and 767 to add longhaul service to the system. That can be read that the 321s will alleviate the 757s that are currently being used because of their capacity on Florida runs, so that they can be better used on transcon runs where their range is needed.