This is the key.
If you're not raising your heart rate, then you're not building any cardiovascular strength or endurance. Yeah, a long leisurely stroll is lovely

but after a while, it just doesn't do anything else for you. Your body will adapt to the exercise and you won't see any more progress from it.
It's not necessary to exercise to exhaustion - 2 hours of walking until you're ready to fall down can be counterproductive.
30 minutes of moderately intense exercise (again, not to exhaustion) are going to be more beneficial.
One of the best things I ever spent money on was a heart rate monitor. I got a Polar F5 (at Target). I wear it whenever I exercise, whether it's just walking, going out rollerblading, or working out at the gym. I make sure I push my heart rate up to at least 65%-70% for a solid 30 mins. I also use it for HIIT (High Intensity Interval Training) and it's made a world of difference.
Hope that helps!
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