Walking is great, but I'd be going for a bit longer than 30minutes if you can find the time. If you can set aside 45 min to 1 hour you will see a better result more quickly and get more toned and fit. You can always cut the time back once you get to your goal weight maintenance rather than weight loss program.
It is useful to have a gym membership not just to mix up walking with some resistance training but also for those days (too frequent where I live) where it rains and makes outdoors walking unpalatable. This year, we had rain almost every day for 3 months
and I still managed to walk 3 times a week on the dreaded treadmill and do my weights the other 3 days.
If you are a mesomorph body shape, you can really take your pick of exercise, though a 1/2 and 1/2 balance of cardio and resistance training is ideal. If you are an endomorph, more brisk walking and less weights will work best for you. If you want to read up on body types, I've got some info on my website that could help you with exercise and diet options.