Workout diversity - running and elliptical training

It's been a long time since I've been consistently active and I have to be careful about over training. The elliptical machine is a great alternative to a daily run and I've discovered that setting the resistance to 3x requires an exertion similar to running on the treadmill. The advantages of the elliptical is that the burden on my knees and feet is much lower and the arm motion, especially with resistance, provides some upper body exercise. The elliptical does not provide a consistent aerobic challenge so I find I can go longer but I don't feel like I'm helping to improve breathing and lung capacity. During my days off I did some longer sessions, 25 minutes on the elliptical followed by 25 minutes running on the treadmill. Those sessions definitely provided a more rounded workout.