Overall in a general sense, I think the guys are right about the modern age. Most girls like muscle (but they don't want too much muscle either), and I do think that you'll get better results with more muscle than as a skinny guy. I'm saying this from my limited perspective as a slim guy that still has some definition and tone (I know it's not much, but I can do 7+ pull-ups).
As far as our ancient past, I don't know. Do you guys have sources? I would only think slim guys who could run long distances would be seen as alphas in particular tribes that valued those traits. It would really depend on the tribe. Else, a swift hunter may be more beneficial... or a man with knowledge of how to catch fish... or just more muscle in general to beat up all of the guys wanting to be alpha. I think in our ancient past that more muscle would still be more favored by women compared with very slim long-distance runners overall. Slim also implies that you're malnourished. If you're the king and alpha, you just get slim guys to hunt and feed you haha. But, I don't have enough data to debate this.
You can still have muscle and be a competitive runner; you just won't be competitive in the Olympics. In my community, there's a guy with gobs of muscle that runs a sub 2:30:00 marathon! It's crazy. In your workouts, do more weightlifting than running. For example, you would lift weights 4 times a week and run 3 times a week. Also, you should lift weights before running. If you do a run before lifting weights, it will tire you out and make your weightlifting not as effective. You can definitely have muscle and be a competitive middle-to-long distance runner -- at least locally and nationally. Just remember that the heart is also muscle and needs to be worked on as well; at least do some type of endurance workout once a week or so....
Lastly, I want to point out that you can still get girls in running circles. If you can socialize well and do social circle game well, you can just join a lot of running communities, and girls in these communities are actually more partial to slim guys (in my experience), especially if you win or get in the top of the pack. If you're around women that are used to the body type and love the sport, they'll also love you.
At the end of the day, you need to judge what you love and care about the most. If you love middle-to-long distance more than life, then keep doing it. But, if it's not that important and you want to be with more girls, then start adding muscle. Muscle is not 100% necessary though, and just because you get it doesn't guarantee every woman you see will fall in love with you. In fact, if you work on your dating skills while you're skinny, it'll be more challenging and make you even better at it until you add muscle. It just depends on what you want in life and how you want to get there.