The situation as presented provides more than adequate information to provide specific 'starting points', and I never implied that they are more than that. It is common; perhaps routine that endurance athletes comsume a higher percentage of calories from carbohydrates (and yes he may need to eventually push it up to 60%).
If he had asked if people think it is possible to add muscle while maintaining that level of conditioning work, my answer would have been different. Although you and I agree about more than we disagree(I don't like zercher stuff either...LOL); our styles are very different. What you frequently call a limit, I'd call a target or short term goal. We are all different, but we aren't that different.
The advice I provide has proven to be effective over and over again, countless times. Ultimately the goal is for the individual to 'become intimate with themselves' and take ownership of the program. I stand behind my suggestions. Nothing radical in my advice to the OP, just basic 'starting line' stuff.