The "Ideal" Amount of Exercise

My co-worker Joe was recently talking about his high blood pressure.  I asked him whether he exercises, to which he jokingly replied "yeah, to and from my car".

He then mentioned that he read somewhere that walking 20 minutes a day is significantly beneficial for health, and that he has a friend who started doing this daily, and lost a significant amount of weight as a result.

I thought that it's interesting if doctors are recommending just 20 minutes a day of walking now.  Not too long ago I remember reading that 60 minutes a day of aerobic activity is ideal, then more recently reading a recommendation of 30 minutes a day.  For a while, I was utilizing the 15-minute workouts that Men's Health frequently posts on their website.  They also often promote a book called "Big Book of 15-Minute Workouts".  I'm guessing that doctors and trainers are realizing that the loftier the goal is, the less likely that people are going to be to meet it, especially lazy and obese Americans.

So what is the ideal and/or sufficient amount of exercise that we should strive to get then?  Is it 60, 30, or 15 minutes or aerobic?  What about strength/weights?  Or should you just walk for 20?  Obviously, more activity is better.  And a combination of strength & aerobic training is also ideal.  However, if you are having a hard time finding an exercise routine or it keeps stalling out, then instead of getting overwhelmed with high expectations and lofty goals, just start with the simple idea that the ideal amount of exercise to get is this: 

more than you are getting right now.  

That's it.  Just start with whatever you can possibly fit in to your schedule and make it happen.  Then do a little more.  Then perhaps a bit more.  But to start, just focus on doing more than you're currently doing.