Does it really matter?

What should you do first, cardiovascular endurance exercise or weight lifting? Or does this even matter?

The answer really depends upon what you are trying to accomplish.

There isn't any special magic in exercise order, but some reasons it may matter have to do with the available energy for exercise, the causes of muscle fatigue, and the risk of injury during exercise. You will always hear different answers why one is better start with than the other. In general, exercise you perform when you have adequate energy is performed at a higher intensity with more focus and efficiency. Exercise you perform when your energy supplies are low is less effective and more likely to result in injury.

If Your Goal is Improving Overall Health
To improve overall health, it really doesn't matter if you lift weights first or do endurance training first. In fact, you can do both at the same time with interval training or circuit training routines or you can alternate weight lifting and endurance days if you prefer.

If Your Goal is Increasing Cardiovascular Endurance
In order to build and maintain cardiovascular endurance, you should perform endurance exercise first, when you have plenty of energy for long-distance exercise. Add resistance exercises 2 to 3 times a week, either after or separate from the endurance work in order to develop muscular strength and reduce your risk of injury. Lifting prior to running is not recommended because you increase your risk of injury due to muscle fatigue.

If Your Goal is Increasing Muscle Size and Strength
Increasing muscle size can best be accomplished by lifting weights first when the body's main source of energy for muscle contraction (glycogen) is high. If you do a hard cardio workout before lifting, you deplete glycogen, which makes the workout ineffective.

Some people do there cardiovascular endurance training and there weight lifting on alternate days which works great as well. I like to do Cardio first because that is my emphasis where I am a runner and I am training for a marathon. I usually pair my cardio with my legs Mon, Wed, Fri, and with my arms and upper body Tues,Thurs, Saturday.