I'm so very sorry to hear this news. It's been so long since I was here that I couldn't remember my password, but there's no way I could read but not post. Cathy was just such a warm, loving woman. My sincerest condolences to her family and many friends.
Rather than swimming a distance, I'd recommend taking more of an intervals type approach.
How about looking somewhere like Zoggs Swim4fitness (free to register and you get a series of swim sessions tailored to your goal).
First things first, though, be sure your technique is good - swimming's not like other sports, in that you can't get away with dodgy technique! (At best you'll find it much much harder work than it should be, at worst you'll end up with an injury)