Quote:
Originally Posted by
hendrix 
marathon running is completely different, since after about 2 hours, all glycogen stores are depleted so the body physically has to rely on burning muscle/fat and getting some sugar input. or at least that's what i've heard.
In general, but remember that those skinny Kenyans are doing marathons in ~2 hours - those last few minutes aren't what's wiping out all their muscle mass.
It's not running every day that's the problem, it's how you run - how far, how hard you're pushing. A once-a-week 20-miler won't help gain muscle mass. An everyday 15-minute trot covering 2 miles won't hurt.
A person interested in both should train and find his own balance. Picture a Venn diagram - the blue circle is time spent running, the red circle is time spent weight training - the white rectangular area surrounding both is your recovery time (and the older you are, the more white space you'll need). As long as both circles are fairly small and surrounded by lots of white space, you're fine. As the circles get larger, as your workouts get longer or more intense, your body has less and less recovery time. When the two circles start to fill the page, gains will stop because there is not enough recovery time, and recovery is when you make gains, not when you're training. When they overlap, you'll make losses, not gains, and you've got a purple area called the Injury Zone.
Your options are to reduce both circles until you've got more white space, reduce one circle to make more room for the other, or find the Fountain of Youth in hopes of adding more white space.
Everyone's mileage will vary - keeping good logs will help you find your balance. From there, decide which circle you want to be bigger, or if you're happy with them being even. Personally, there's only so much lifting I want to be able to do - once I'm hitting my weight targets, I work on my running. When I can't hit my lifts, I ease up.