So what about the numbers and stats?
Run equivalent miles: 1/4 mile swim equals 1 mile of running. Thus a 100 meter swim= 400 meter run. Try a pool interval day for change of pace! A 1.5 mile swim= 6 miles of running!
Heart rate: A 1999 study published in the "Journal of Sports Medicine and Physical Fitness" shows that heart rate is typically 15-20 BPM lower than running. This is due to being in a prone position, and in a buoyant environment, both allowing the heart to pump more blood even at a lower heart rate. Thus, don't expect your heart rate chart for running to equal that of swimming workouts. If the pool is warm- 86-95 degrees, your heartrate will be closer to that when running on land.
Calories: Be prepared to eat more if you swim compared to running. It is estimated your burn 25% more calories swimming compared to running. This is due to the density of water and the muscle work needed to propel yourself.
Bottom line as a running coach, I would talk to you about adding swimming in your workouts a couple of times a week, particularly in your build up phase and then as ancillary training on recovery days or as a 2nd workout to virtually any run day, or if you just aren't feeling like a run one day, go jump in the pool!
Jeff Bangle, DVM prrunningcoach.com