Why does endurance exercise interfere with muscle growth?
Image from Exercise Physiology, Theory and Application to Fitness and Performance by Powers and Howley.
In the figure above, endurance exercise results in a signaling cascade in the muscle driven by the depletion of ATP among other things. ADP and AMP accumulation cause the increase in AMP Kinase, and then TSC1/2 downstream. TSC1/2 inhibits mTOR. mTOR is part of the anabolic signaling cascade in muscle following strength training. This is presumably how endurance exercise training can interfere with muscle hypertrophy and why marathon runners are so skinny (continually supressing mTOR).
The real question becomes how much and at what intensity is required to inhibit mTOR? This is still up for debate and to some extent it is probably an individual response kind of thing. Some people seem to be able to handle lots of endurance exercise without losing muscle.
During really easy exercise (i.e., walking) the AMPK response would not be very high. During brief, but intense exercise, the same is true. It is the middle ground, moderately long, moderately intense, that probably produces the biggest AMPK response (i.e., tempo run intervals or prolonged zone 3 pace efforts, or half marathon pace).