I actually find it a bit troubling that a lot of these "old fat guys" are simply relying on carrying a firearm to be the only tool in their inventory for self defense. Even if you were to practice regularly, I think it's very narrow minded to think that threats will only be that 5-7 yards (or whatever) that they are statistically, and approach it as such.
What if you did have to run a hundred yards (combined), perhaps jump/climb over some things, etc. What do you think you're heart will be doing when it comes time to pull the trigger? Your rarely used muscles will also be fatigued. You could suffer from serious accuracy issues, or worse, begin to suffer from a heart attack or stroke.
I guess my concern parallel's some of the other members recently; people aren't thinking outside the box. Many people have made the first step of shedding naivety by acknowledging that there are bad people are out there and it is necessary to be prepared for an encounter with them, wherever that may be. However, too many make the mistake that they will always have the drop on the bad guy, that even though the bad guy has his weapon out first, you will be able to draw before he can react. That the bad guy is dumb, and therefore easily defeated. That being fat is ok... because you have a gun.
If not for your performance, at least do it for your health.