Preventing Muscle Loss While Training

I am currently training for a half marathon in February.  I lift pretty heavy during the winter using olympic and combo sets to build power and strength for the upcoming tri season.  However, I got to thinking that with all of the running I will be doing, I may lose a significant amount of muscle.  I've seen some pretty jacked up triathletes, both men and women, and was wondering how they retain all of that muscle with the level of cardio that they're doing.  Is it genetics or are they just putting in an amount of weight training that is comparable to their s/b/r training times and effort?  Also, is there any truth to the information I've read on body builder sites that states that extended cardio sessions will often result in a loss of muscle?  I have a hard time believing anything that could be based on Bro Science.