Breathing

Hi, I started swimming a couple of months ago following a running injury. I can't run at the moment so I thought I would keep fit by swimming, but my problem is I don't know how to breathe... I can barely do 25 mt and then I have to stop and rest for 30-60 seconds. I have no problem running (I completed my first half marathon in March) so I can't understand why this is so difficult.. I'm working with a teacher to improve my freestyle crawl technique (which was quite poor as I hadn't had much practice in the last 20 years or so...) and she says the breathing will come naturally, but after 2 months swimming twice a week I can't see any improvement... still cannot swim continously for more than 25 metres. I can do *** stroke or back stroke without stopping, it's just the freestyle what gets me completely out of breath. Is there anything I can do to improve my breathing technique? Thanks