I don't doubt that activity is beneficial. It makes sense, and we all try to be active.
But, don't you sometimes wonder if the study might be confusing cause and effect? Maybe the folks walk or exercise vigrorously because they feel healthy and have a lot of energy. That's why they have a better survival rate: they're healthier individuals to start with.
On the other hand, isn't it possible that the group who doesn't exercise avoids exercise because they aren't very healthy and aren't feeling well. Naturally, they don't live as long because they were unwell to begin with.
When I feel really good, I exercise a lot. When I don't feel so good, my activity level drops.
OK, I know that's backwards thinking, but sometimes I just wonder about these studies.