That's great advice, but it's a pretty big task for someone who might not get a lot of exercise to do 2 hours of intense weightlifting. I lift regularly, and my gym sessions only take an hour.
The good news is that if you are sedentary, older, or sedentary and older, even a little bit of exercise will be extremely effective. Go for that long walk, or maybe a shorter jog, and do some pushups and un-weighted squats. You'll be exhausted, and you'll be less likely to injure yourself. :)