I'm good friends with a number of long distance runners at school. Made the mistake of running with David one morning before I knew what he did... my goodness is low days are 7-8 miles. Most of them are ripped to shreds, but don't have much upper body mass, not huge amount on the legs either. Two reaons.
Big upper body helps for starts, thats true, but once you get past 1/2 mile runs you don't come out of blocks anyways, so usually spriters will have the bb look. Look at michael johnson, he could bench 225 20 times, and did so weekly. His split time on the 200 would/did beat the old record for the 100 mt.
Longer runs even fast starts don't play as much of a role, and that 10/20 lb of muscle is just a hinderance so they are skinny lil freaks
If he's looking to do marathons he's gonna be heart broken like a 6'5 110 lb guy would be when his first goal in lifting is to beat out Coleman, drug free. Tell him to set achievable goals, ones he can reach in a month or two or so. And build up to marathons.
Lots of pasta, stay away from simple sugars the burst isn't good for long runs. A good recovery shake post run is essential.