Sure you will burn more calories!!! It takes your body about 30 minutes to get into the "fat burning" mode so anything beyond that will produce fantastic results. I try to do 60 minutes of walking each day; but do whatever you can! Many times I even break it up into 2 30 minute segments; it just depends on my schedule! I'm glad that you are taking steps to becoming a healthier you!!! Keep up the good work!
Another alternative is, rather than doubling your time, boost your speed. As you're building up your speed, you'll still need to spend more time, but the amount of time you'll need to burn the same amount of calories as an hour walk will go down.