This is possibly the most universal exercise ever and it can be very effective. You can walk on a treadmill or anywhere outside. You can even use an elliptical machine (or a stepper for a really intensive cardio). Start by walking 2 minutes at 3.5 mph. Always keep your abs engaged for proper posture. Then speed up a little bit for another 5 minutes. Then walk for about 8 minutes at an 4.0 incline. If you want, slow down a bit. Increase the incline again and maintain the same speed. If you feel well, try and jog a little for about 10 minutes. At the end, go back to walking at zero incline.