Combine strength days with cardio days
The more muscle you can get working, the more it will challenge your heart and your cardiovascular system. Instead of building cardio-only workouts (the pitfall that’ll prevent you from building endurance) make sure to weave strength days into your training. Most people reserve one day for strength and another day for cardio. Try combining the two instead. Use a bench press, immediately followed by pull-ups, then run a mile as fast as you can… and repeat. Another good example: Thai Rope for a minute, followed by squats, an overhead press, and finally Plank Repeat.