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I was communicating with an athlete I coach recently who is focused on being at his best for an open-water swim race. He wanted to know what he could do to get faster swimming; he told me he could not get to the pool as often as he wanted. I told him to simply train more—including cycling and running!

He said: "How will cycling help my swimming?"

The primary determinant for aerobic fitness is simple: total training time. If your aerobic system becomes more efficient you will go faster in any sport that relies on endurance, and that includes any activity that goes for more than ~2-minutes. Under ~2-minutes is sprinting, a different energy system (anaerobic).

Another athlete I know was focused on running a quality Boston Marathon. She qualified at an end of season marathon where she had been training for triathlon all year, then decided to run a marathon. After qualifying her training shifted to nearly all run work with no swims and just a few occasional rides. Her race at Boston was much slower than her qualifier effort—what happened?

When she raced the Boston qualifier she was training about 10 hours a week including swimming, cycling, and running. Then with her focus on Boston she was actually running slightly more than during triathlon season, but only training about 5 hours a week total. Her aerobic fitness base deteriorated with half as much training time per week.

Bulk of training time works. This is why elite swimmers will spend 4-hours in the pool every day: elite bike racers will ride up to up to 600-miles a week; elite runners will put-in 120-miles a week or more. I know pro triathletes who put in 8-hour training days!

Unfortunately the gains are not proportional to time spent training, but they are significant. With repetition your metabolism becomes more efficient. Your heart gets stronger and pumps more blood with each stroke. The energy processing cells (mitochondria) convert carbs and fat to muscle fuel (ATP) more effectively. When you repeat a muscle movement the economy of that movement improves; you move forward at the same speed with less energy.