Exactly a year ago, I realized I was in horrendous shape (I knew it wasn't good but I didn't realize it was as bad as it was). I had been doing the treadmill thing - walking briskly (my knees don't take kindly to running) about 20 minutes a day, 4 days a week. But that wasn't doing my cardio any good and wasn't getting me any weight loss.
Once I realized how bad things were, I started doing a bit of research on the internet. I bought myself a cheapo polar heart rate monitor. I don't think they make the one I bought anymore, this
Polar F1 is similar. Over the next couple weeks I increased the length and intensity of my workouts, until I was doing 45 minutes 4 days a week.
This site discusses fitness walking and this link in particular
Target Heart Rates and how they are used for weight loss.
Once I increased the length and intensity of my workouts my weight started dropping and my heart got more efficient. I dropped about 27 pounds in the past year and got my bodyfat percentage to a more reasonable 17.5% (still could use more work there). Most of my weight loss was in the first 6 months. The last half year has been fairly stable, although I expect that diet improvements would yield further good results. Although I eat healthier than before that is one area that still need a lot of improvement for me. But I don't much like cooking or taking the time to prepare lunches, so I eat out far too much for lunch and don't eat as well as I should for dinner.
Site I found helpful:
The Walking Site
Rockport Walk Test
ExRx.net
MyBodyComp.com
GOOD LUCK!!!