I've said it here millions of times and I'm saying it again - it is amazing how resistant distance runners are to weight training despite the overwhelming scientific evidence of improved speed/running economy and the fact that literally every athlete in other sports and track events do this.
Just calling this out because someone needs to say it - I am almost certain this is due to fear of "getting big" as body image issues (and subsequent eating disorders) is endemic in the sport. A harsh prognosis, but reality hurts sometimes and the truth will set you free.
New Flash - it's harder than you think to put on weight/muscle if you're not actively trying by eating more, which is pretty hard if you're already 1100 calories in the hole from your daily ten miler. You may not gain an ounce of new muscle, but the ability to apply more force into the ground through CNS improvements alone will make you a stronger, faster, and more resilient runner. Again, the scientific evidence is overwhelming in this regard.