Moderate regular exercise, much like calorie restriction, is demonstrated to slow down almost all measures of aging investigated to date. It's fair to say that exercise modestly slows aging - or perhaps more accurately we would say that a lack of exercise accelerates the accumulation of biological damage and resulting decline in health that occurs with the passing of years. Being a sedentary couch potato cuts short the life you would otherwise have lived, and makes its later years much more unpleasant to boot.
We can talk about some of the ways in which exercise impacts long term health and the state of your biochemistry. Given that aging is nothing more than the accumulation of damage, it is probably important that exercise has a hormetic effect: it puts a little stress on the system, which activates heat shock proteins and thereby boosts cellular repair and recycling mechanisms - such as autophagy - to greater efforts, producing a net benefit. This may explain why exercise is shown to produce lower levels of mitochondrial DNA damage, and hopefully you all know by now that mitochondrial DNA damage is one of the important contributions to age-related degeneration.
Today I noticed another speculative mechanism by which exercise can modestly improve your lot in life. You might recall that one of the primary issues with our immune systems is that necessary cells become crowded out over time - the body supports only so many immune cells, and the naive T cells needed to fight new invaders become depleted in favor of memory cells uselessly devoted to persistent viruses that the body cannot clear. In essence, the adaptive immune system is evolved to hit the ground running and fight the battles of a young life - and that front-loading of its effectiveness leaves us high and dry later on, once too many battles have been fought.
One of the strategies that medical researchers could use to solve this problem is to destroy the unwanted specialist cells, freeing up room for more useful T cells and restoring immune response to a more youthful level. The targeted cell-hunting and cell-killing technologies under development in the cancer research community would be ideal for this use.
Now consider that even though the vast majority of people are infected with persistent viruses like cytomegalovirus, the quality of their immune systems in later life varies widely. Some people have comparatively good immunity when they are old - nowhere near as good as when young, but certainly better than their fellows. It could be argued that exercise goes some way towards establishing this difference - and in this paper, the argument is that exercise lures out decrepit T cells so that they can be destroyed by the body's maintenance systems:
Overcrowding the "immune space" with excess clones of viral-specific T-cells causes the naive T-cell repertoire to shrink, increasing infection susceptibility to novel pathogens. Physical exercise preferentially mobilizes senescent T-cells from the peripheral tissues into the blood, which might facilitate their subsequent apoptosis and create "vacant space" for newly functional T-cells to occupy and expand the naive T-cell repertoire.
Bear in mind that exercise is no substitute for the biotechnology of the next two decades - people who regularly exercise become frail and age-damaged in the end as well, they just tend to have a longer span of healthy years and a longer overall life expectancy. You can't exercise your way into rejuvenation, no matter what the fools in the "anti-aging" marketplace might say on the topic. But if you want to make the best of your chance to live to see the age of true rejuvenation medicine, you should take good care of your long-term health in the obvious and straightforward ways that are known to work.