I'm not an expert, but have spent the last week reading up on walking.

If you do an internet search on "power walking" you'll find lots of useful info. The sites I've found say that 12-15 minutes per mile is what you should be striving for. It will take a while to get there. You're doing about 20 minutes a mile. That's the speed I'm at right now, and it's fast enough to break out into a sweat at.

The biggest tip they have to increase your speed is to make your strides faster, not longer.

The sites also say that power walking (or fitness walking) is as good a workout as jogging, with less strain on the knees.