Power walking is cardio. You might look into interval training. I've been reading that studies are showing that interval training does more good than intense exercise for long periods/distances. That means alternating the power walking with regular walking -- say, 3 minutes of each.
It's also good to add weight training to your routine. Maybe do 1.5 hrs of walking and 0.5 hrs of strength training. But all kinds of exercise are good for your body.
There is no doubt that power walking benefits you. It is a good form of exercise. It will help with your blood circulation and will help keep the extra weight away.
If you can add more cardio to your routine, that would be even better.