Don't take this as directed at you Bill or as an insult. But IMO people need to be careful that they are not simply justifying their fitness decisions (or lack thereof). It can be perceived as an excuse for the status-quo. You are engaged in fitness training so I don't think that you fall into that category. I think that the OP's last sentence is important.
Thats the bottom line for me as well. Nobody is saying that all martial arts practitioners need to engage in a pro-MMA conditioning program. But they should be doing something.
But I don't want to be 'as fit as I can be given my circumstances'. I want to be 'as fit as I want to be.' The difference may be subtle, but I think it's important. Life is about risks and rewardes, pleasure and pain, and mostly, about balance. I choose the level of effort I will put forth to gain the results I most desire; but I will not neglect the enjoyment of my life for an extra lap in the pool or mile on the treadmill; nor will I be led or allow myself to feel guilty or bad or wrong about those choices.
I take it as a given that I will not be able to run as far, be as flexible, or fight as well as someone who devotes more time and effort to it. I give a lot of my time towards this art because I want to; but I also stop when I want to and simply enjoy my life. Might I lose a self-defense fight because I ate a doughnut or didn't run an extra mile? Sure. Risks and rewards, pleasure and pain, and in the end, balance.