Base swim workouts for the winter?

I'm looking to add some structure and purpose to my swim workouts over the winter. I've only been swimming for about 6 months total...maybe 2x per week. When I started, 50m was all I could do without taking a 2-3 min break to catch my breath. Now, I can do a mile non-stop...but it's a little boring. I decided to focus on swimming over the winter and take a few months off from running...I've been a runner for 15 years. I plan to swim 2-3 times per week for about 60-75 min each time...I'm limited by the lap swim schedule at the local pool. Up to now I've simply been making up a workout without any real knowledge of how to structure it for improving my swim. Since I've only recently started swimming I figured just getting the laps in would be sufficient to build up my basic endurance. Now that I can comfortably do the distance I'd like to start incorporating specific workouts to improve my speed and efficiency. Most recently I've been doing 3200m pyramids or ladders that take me about 60-75 min with several 1-2min breaks...200, 400, 600, 800, 600, 400, 200 or 200, 400, 800, 1200, 600 for example. I'm looking for some advice on how to structure my winter swim workouts with the goal of competing in mostly sprints with a couple Oly distance events next season. Open water swims aren't an option since it's getting cold and I don't have a wetsuit yet.

Thanks for your input!