Even if you haven't been as active as you'd like during your working years, that doesn't mean you can't get moving after retirement. In general, people actually tend to become more active after they retire.
In one study, women raised their moderate and vigorous activity time by 31 minutes per week. Men increased their activity by 42 minutes per week. So get up, get out there, and enjoy! Exercise not only feels good, but it's good for your health, too.