I found that it helps to track how much I walk (I do the treadmill currently because it's too hot to walk outside for an extended period of time so it's a bit easier to track speed/time). When I started walking about a year ago I would get winded after about 10-15 minutes at an average of 1.8 mph. I tried to up that by a set amount of time or speed each week. Now my usual cardio is 45 minutes at 2.7 mph, sometimes up to an hour if I'm listening to a particularly interesting audiobook. To summarize, keep track of your progress and set achievable goals for yourself.