It's not that big of a deal. He's not a competitive marathon runner. THOSE guys have problems hanging on to muscle mass. For everyone else, cardio shouldn't even be a concern when it comes to muscle mass.
There's been some kind of crazy myth floating around, probably starting with bodybuilders, that cardio kills muscle mass. It doesn't. Muscle mass is simply a response to a training stimulus. Many people that are overly focused on cardio don't lift weights at all, or don't lift heavy weights enough. Well, of course that's not a recipe for maintaining or increasing muscle mass.
As for the cortisol issue, directly correlating it to cardio is incorrect. ANY overtraining, whether it's cardio or weights, will increase cortisol. In fact, that's where the post-workout carb thing comes from. Weightlifters wanted to reduce cortisol levels after lifting, and so spiked their insulin. That's one way to do it, but it's not as big a deal as they make it out to be.
The key to controlling cortisol is rest and proper nutrition. Eat good fats and plenty of protein and veggies. You can't overtrain, whether you do it through cardio or through lifting weight. For people with lots of excess body fat, cortisol can be very helpful, as it mobilizes the transfer of energy from adipose tissue (fat) to the bloodstream. Can it cannibalize muscle tissue? Sure, but if you get enough protein and are subjecting the muscles to adequate amounts of training, it's not going to happen.
There are plenty of athletes that do more cardio than the average gym-goer and still maintain and build muscle. Even the ones that aren't on steroids
As for muscles burning more calories, it's not as much as you'd think. One pound of muscle only burns about 5-7 calories per day. A very insignificant amount.
http://www.acefitness.org/fitnessqan...309&itemid=358
Now, I agree with you that 30-45 minutes of cardio is enough, but it's intensity that matters, not length. That's why I've been hammering on this HIIT concept. Steady-state cardio like running 3 miles, doing 4 miles on the elliptical and some on the bike isn't great, for many reasons. It's not really that intense. Steady-state cardio can increase carb cravings and cause you to eat more. And finally, HIIT is just a better time investment as far as calories burned per minute goes. Now, if he's doing all of those 12.5 miles as HIIT, then I have no problem with it. But 12.5 miles of steady-state isn't nearly as good for you as the equivalent time in an HIIT program.