The best way to get that cardio in through weight training is to lift weights faster! Not the kind of fast you are thinking…….if you were thinking of yourself doing bicep curls as if you were in a movie that you are fast forwarding, that is not what I am talking about. You will more than likely get hurt doing it that way.
To get a cardio workout in at the same time you are weight training a good approach would be to try CIRCUIT TRAINING. Circuits are performed with no rest between exercises. Rather than performing all your sets with one exercise before moving on to the next exercise, you would do your exercises in a circuit- one set of each exercise on your list with no rest between the moves. So take 4-5 exercises and do each one for a set number of reps and go right on to the next exercise and so forth. Catch your breath and rest after you have completed every exercise on your list one time through. Then repeat the list of exercises.
Doing exercises back to back with no rest will get your heart rate and breathing rate going and this is considered cardiovascular training as well. This type of training also allows you to get the most out of your workout when you are pressed for time. That's a win in my book. And if you still crave that cardio and just don't feel the weights are enough then lift heavier! OR- you can do a lighter weight but do more reps, about 15-20 reps per exercise OR-you know I love my "ACTIVE REST" do something between your circuits for 45 seconds to a minute; jump rope, high knee run, mountain climbers etc….these will help increase endurance, lean muscle mass and metabolism. But hey, if you have the time for a cardio session and love the slow pace and focus of a weight training session with no rush, then great, enjoy!