That's difficult to specifically answer w/o more information. I'd suggest you start off (if you haven't) by figuring out your daily caloric needs (search BMR-basic metabolic rate-on a search engine)...you can also find sites that list the estimated amount of calories you'd burn...based on gender, height,weight etc..by activity and level of intensity. That way you'll know what you need to take in everyday to be healthy...what you've 'spent' everyday in exercise...and you can adjust your intake to be slightly less than your 'needs' to result in a weightloss.
You should seriously evaluate your currrent diet..is it more than 20-25% fat? If so...a dietary change...while maintaining your current exericse might be enough...just some thoughts
You can't spot reduce through diet and exercise. If you're trying to lose fat your body pretty much takes it from all over. With this in mind, a pound of fat is approximately 3500 calories. In order to lose a pound of fat per week you'd have to operate at a caloric deficit of 3500 calories for the week, or 500 calories per day. You can do this by diet, exercise or both.
Walking and running burn calories at approximately the same rate, about 100 calories per mile. I would continue walking as you are, but increasing the distance as you feel your fitness level increase. Another good way to lose weight and/or tone is to add weightlifting workouts 3 times per week.
Thanks Indyman...and thanks for that link...I'm definately gonna check that out!
Also in addition to what you had said earlier...to help illustrate the importance of strenght training as part of a sucessful weightloss plan....each pound of muscle mass you can add to your body will basically increase your BMR by approximately 50 calories per day...so it helps to burn off the weight even faster....also there are an increasing number of studies that show that your metabolic rate will stay elevated for a longer period of time (like nearly 24 hours) following weight training then following an aerobic workout (like an hour or so) which also helps!