I've decided to swim a 2.5km race a year from now. For the last two years I have swum 3km daily, but at a uniform, comfortable pace which I believe is not optimal for boosting competitive performance. I wonder what swim practices should look like if I'm interested in getting really good at 2.5km freestyle? Also take into account that I swim every day, so maybe I should alternate between practice sets?
I used Triathlete Magazine's Essential Week-by-Week Training Guide: Plans, Scheduling Tips, and Workout Goals for Triathletes of All Levels for training. For the swim portion there were a lot of drills for improving technique. It was the author's opinion that improving technique would improve speed. (That said, the best thing that improved my technique was two sessions with an instructor.) There was also a lot of fartlek work: Sprints coupled with recovery. For example one day's swim would be 50 m as fast as I could go with 150 m at a moderate pace for recovery. Repeating that several times along with some drills for technique would be one day's training.
Please tell us how your race went!
But to answer the question, to become good at long distances like the 2.5k, you need at least two things. You already have one of them: you have done the distance in practice. The second is to raise your aerobic capacity. So those "sprints" that triathlon magazine you got a tip about in another answer are really not the way to go. Sprints are anaerobic, and although you will need some of that in open water swimming too, it is not the core of your workouts.
Here are three examples of sets you can vary and use to get your heart rate up to just below your anaerobic threshold.
- 16x50 at an interval that gives you between 5 and 10 rest.
- 10x200 at an interval that gives you 15" to 20" rest.
The point here is the interval. Don't just rest a set number of seconds, force yourself to make the chosen interval and to get more rest you have to swim faster.
- 2x50 + 100
- 4x50 + 200
- 6x50 + 300
- 8x50 + 400
- 10x50 + 500
Notice how that set is 500 longer than your race, and the same as your current workout. The 50s are for getting your heart rate up, faster than your race pace, and the following distance is for you to practice your target race pace.