(if your life expectancy increases, your extra years may be spent retired, on the other hand, you're more likely to be ill in those extra years, etc.)

I'd expect most of the life expectancy-increasing effects of exercise to also shift the age at which you'll get ill by almost the same amount, while hardly affecting the age at which you'll retire, so I'd guess most of the extra time would indeed be while you're retired but still healty enough.