First open swim, need workout plan help

Hi all, MY 1st post! I'm thrilled I found this site. I hope you can help. I'm a good swimmer (12+ years of year-round competition when I was a kid), but have taken a LOT of time off. I'm 34, been back in the pool for about a year and feel great! I've been doing about 2000+/- yd workouts, some easy long sets, others short quick, little rest time. I have been staying in shape otherwise -- running, weights, yoga. Here's my question: I'm doing a 1-mile open swim in June. I'm clueless about workouts. I've never trained for a distance like this. I can do a mile in a 25-yd pool in about 22 min -- pushing it hard. But I'd like to improve that time. I dont' know how this translates into open water time. And I don't know how to train for this. I'd really like to be able to run, lift and yoga as well. I'm looking for a 3 day per week program (is that enough?). I just want to get my speed up. I'm perfectly comfortable with the distance, so I'm not concerned about that. Any ideas would be very much appreciated! Thanks so much.