You make some good points. Reason? I guess it's part "I've always wanted to run one" and part challenge to myself. I know that running that level of distance is a significant mental challenge and I just kind of want to have experienced that, for better or worse. I suppose that may not seem like the best of reasons, but I guess why does anyone do anything?
As for the loss of muscle and drastic reduction in overall fitness, that's kind of what I was thinking was avoidable and why I posed the question. There was a CFJ article recently about Greg Amundson attempting a 100 mile run off CF WOD / Zone alone and getting like 80 miles or so (and probably doing serious damage to his body in the process). I'm not Greg Amundson in a lot of ways, and I certainly don't have access to the coaching / resources that he does re: running technique, rehab specialists, etc.
Maybe I'm overshooting, and maybe I'll find that it's not for me sometime in the training. I guess i'm seeking the best marriage between the world of overall fitness and the ability to do the distance effort.