Ironman swim workouts

Hi. I am new to swimming. I finished IM Wisconsin's swim in 1:17. I followed the TI principles and started swimming in Sept 2003. I averaged 1.8 swim workouts since Dec. I feel my form is ok but I need work on my catch and pull. I have a goal of 70 min for the IM swim. I am looking for suggestions to achieve this goal. I do not have a swim coach, as far as I am aware, I my area. What are the typical swim workouts for Ironman Triathletes. I am thinking of swimming 4 times per week (1 2000 yd, 2 3000 yd, and 1 3000 to 4000 workout each week focusing on form, drills, long endurance swimming, and some interval workouts). Your help is greatly appreciated. Thanks Brett