My Answer: There is no specific formula I use for training plan distance to train vs. race distance other than being sure to swim the distance of the race over the course of one or two workouts each week. For a 3-mile race, I'd try to have at least one workout every other week that was that long or longer, and I'd be sure that at least two workouts during the other weeks were 50% to 75% of that distance. More important to me is the intensity of the workouts, or how fast you are trying to swim. I would try to build up to doing sets of swims (intervals with rest) at the goal race pace. Maybe build up to doing 6 x ~1/2-mile swims, starting each slower than goal pace and ending each at or faster than goal pace, with some rest (30-seconds to 2-minutes) between each swim.
I might include a progressions like this for one workout each week over a period of many weeks, depending upon where my fitness was when I started (the workout would still include warm-up, cool-down, some technique work, etc.). I would either try to start each swim slower than race pace and end each swim faster (build each swim), or I would try to do each swim faster than the one before it (descend the set). I would take 30-seconds to 1-minute rest between each swim
- Week 1: 2-3 x 500 with 30-60 seconds rest between each
- Week 2: 1-2 x 1000 with 30-120 seconds rest between each
- Week 3: 3-4 x 500 with 30-60 seconds rest between each
- Week 4: 2-3 x 1000 with 30-120 seconds rest between each
- Week 5: 4-5 x 500 with 30-60 seconds rest between each
- Week 6: 3-4 x 1000 with 30-120 seconds rest between each
- Week 7: 5-6 x 500 with 30-60 seconds rest between each
- Week 8: 4-5 x 1000 with 30-120 seconds rest between each
- Week 9: 6-7 x 500 with 30-60 seconds rest between each
- Week 10 5-6 x 1000 with 30-120 seconds rest between each
- Week 11: 7-8 x 500 with 30-60 seconds rest between each
- Week 12: 6 x 1000 with 30-120 seconds rest between each
Remember to do swim technique work as often as you can, too. It should be a part of every swim workout. It could be as little as 3 or 4 x 50 of stroke drills - even that little bit could make the difference over the long haul.
I hope that helps with your open water swimming workout. Let me know how things go or if you have more questions
Swim On!