Actually based on some reports I read once we humans are living longer these days due to the medical advancements.
But you are right unless things change our lifespan will start to shorten as shown with the increase on the death rate (for starters). But its not as easy to improve our health as just simply working out. If we want to increase our lifespan we have to change our entire lifestyle. We spend so much time working that we don't have enough time to slow down our life and enjoy it. We are always so pressured to do things , and do them quick, that it affects everything. When talking with old relatives, I always noticed the same thing. They lived a peaceful/relaxed life with no rushes or hurries. And even tough they smoked heavily, drank and did things that they were not supposed to they lived between 90-100 years.