According to a research paper published in Fitness management magazine way back in 1998, not very.
I was digging through my pile of fitness and health clippings and unearthed this very telling graph.
I wish I'd saved the magazine but alas I've long since tossed it into the recycling bin.
If you look you'll see that once you hit a BELOW average level of fitness you've done most of what you need to do to survive longer. Once you hit an average level, any more does you little good.
For women, that little bump in the below average road is interesting - that as you get a bit better than below average you get worse. Who knows.
So the idea that the smug person jogging by you everyday on your way to work is healthier than you or that your gloating friend who's hits the gym daily will live longer than you is poppycock.
A reasonable level of fitness is all you need.
And remember, research also reveals that as little as 2, 15-20 minute strength training sessions a WEEK is all that is required of you to get every itty bitty health benefit you need that exercise can bestow.
So, hit the iron folks!