Cross training decreases the risk of injury, yet adds intensity to a program. More calories are burned per week. People get strong and fit faster. Cross training uses more muscle groups. For example, running builds the lower body.
Swimming works the upper body and improves flexibility.
Alternating these sports gives great benefits. And, it prevents overuse injuries. Variety is enjoyable and increases fitness. A variety of activities can be traded with walking and running. Indoor and outdoor cycling help build strong quadriceps.