Run on a flat path, track or treadmill. You can use the 8/2 method of 8 minutes running, 2 minutes walking.
WU: Swim 4 laps (200 yds) warm up.
MS: The next set will be 8 x 50 on 30" rest. This means 50 yards sprint, 8x, each time with 30 seconds rest when you finish. Then swim 200 yards of drills (mix it up) and then swim 4x25 yards fast with 15" rest.
CD: Swim 100 yards easy cool down.