is it normal to not be able to keep a steady pace when you first start running /jogging?

Running requires a lot of endurance. Because you don't run regularly, your body is not used to it. Even though you exercise in other ways and can endure strenuous activities such as p90x and insanity, running is something else and affects your body differently. When I first started running, I had the exact same problem as you. I thought I had weak lungs or something because I couldn't catch my breath when only running a short distance (even when my pace was slow and steady!). What you need to do is run frequently and regularly so that you can train your body to endure that type of exercise. If you go at it like this, your body will slowly adjust. Eventually, you'll be able to run continuously for miles without getting winded (which is where I am now!). It takes a lot of training, but it's so rewarding as you notice your body improving each day.