As trainers, we are often asked, is it better to do cardio before or after I strength train? The answer really depends on what your goals are. We believe in prioritizing both your time and your energy resources during exercise. So if your main goal is to increase your strength, then weight training should come before extended cardio training. This will allow you to have a full store of muscle glycogen (the primary fuel source for strength training) to use for those presses, squats, and lunges.
After an intense strength session, it's OK to do light to moderate intensity cardio, as you will be burning a higher percentage of your calories from fat at lower intensive (about 50-65% of your Max Heart rate). Again, if weight loss is your goal, then the same principles apply - by doing light to moderate cardio after your strength session, you will be able to more quickly convert stored fat into energy, as your muscle glycogen levels have been depleted.
However, if aerobic endurance is your primary goal (example - you are training for a 1/2 marathon or a long hike), then do your cardio first, while you are your strongest and most focused. Just remember to refuel with a carbohydrate and electrolyte enhanced beverage if your cardio is lasting more than 1 hour.
So first, decide what the primary goal of your workouts are (the body does one thing at a time much better than multi-tasking) and prioritize the related activities in your workout accordingly. Of course, enlisting the help of an experienced personal trainer to help you sort this out is also a great idea that will allow to make the most of your exercise time!