I was starting to develop a slight dad bod over the last 4-5 years. Skinny upper body with a little bit of weight around the midsection. Two things fixed it in about 4 months. Increased my mileage from 15MPW to 40MPW, cut out alcohol completely. After 4 months, I was just skinny but with abs. 6' and 144 lbs.
I stopped running 3 weeks ago, no cardio whatsoever and added in pushups, situps, and now full weight routine. I'm up to 147, and 3 weeks of lifting really makes an upper body difference when you're that skinny. I'm still skinny compared to the majority of the population, but definitely slightly bigger and more defined chest, arms and shoulders.
It's not that I avoided weights because I didn't want to gain weight when I was running more, I simply didn't want to go to the gym on my days off of running. I'll likely start alternating days, 4 days running and 3 days gym per week.