You're the lady who has cf, diabetes, pcos, asthma-- is that right? I don't blame you for finding it difficult. All these conditions tend to drag you down. I'd say just go 5 minutes a day on the treadmill for a week or 2 or 3, however long you need to, then gradually ramp it up by five minutes each week. Five minutes isn't too daunting, and you'd be surprised how it helps over time.
IMO it's more important to establish the exercise habit firmly than it is to get some specific timetable. That's how I started, and it has taken a couple of years, but the payoff happens over time. Just take it in tiny steps at first. Good Luck!
Age 48, 5'6" Living with PCOS.
Start: January 1, 2002, 220 lbs., size 24
Current: 163 lbs, size 10 - 12, 39"-31.5"-39.75"
Goal: size 10 in Levi's 515 jeans.