So many of us yogis believe that yoga is all the exercise we need. Is it true? Yes and no. First of all, it depends on the style of yoga you're practicing. If you are exclusive to yin yoga for example, you will need to certainly add some cardio, balance, and strength training of some sort to your schedule. But since yin yoga is only one kind of yoga, and a bit on the extreme end of the relaxing and stretching part of the spectrum of yoga styles, it's not the best example. Most styles of yoga that have you at some point in your practice standing on one foot, doing standing asanas, and also some asanas that you are bearing weight on your arms (e.g. chattaranga, downward facing dog) are excellent for strength training and balance. Styles that have sun saluations or even 'vinyasas' are good for your cardiovascular health too.
That said, unless you are going to do multiple consecutive surya namaskara vigorously for 30 minutes 5x/week, you are not getting enough aerobic exercise. So no, for your body at least, yoga is NOT enough. The good news is that yoga is amazing for you in so many other ways--please, in my professional opinion DO NOT DROP YOUR PRACTICE!
You might consider adding another activity to your regular routine. It doesn't matter what it is, it can be free or join a class. Walk, hike, run, bike, swim, run up and down the stairs, play an aerobic Wii game, Zumba, whatever just get your heart rate up. My personal favorite? Turning on the music and dancing around my house like a lunatic. Whatever your thing is, make sure your intensity is appropriate. You can figure it out by self monitoring and checking in periodically.
The recommendation is to get your heart rate up within the target range for 30 minutes 5x/week. It involves taking your pulse and knowing where your target heart rate is. You can learn about your target heart rate and how to measure it from the American Heart Association.