To answer the original question, you really need to simply ask if the female is a runner or not. You guys are making this way more complicated than it needs to be. If you are a runner (skinny or not) if the female is equally into running, it will likely impress her. I find it impressive when a guy is faster than me and even more attractive than just a hobby jogger. For a lot of females, they have body image issues so if you weigh less than they do, they won’t like that. That’s the predominate reason, from a female perspective, girls tend to prefer a little more muscle on their man. Now I think any man that commits himself to making his body a slave to his training is attractive since it shows work ethic. But I am not a typical female either. I focus a lot of my time, energy and resources on my running so a male runner who does the same will be attractive to me regardless of his physical appearance. Potential training partner/buddy? Understands I would want to have a rolling recovery date vs fancy night out? - all of these potential qualities would be impressive to me. And - If a guy lowered his pace by 30s per mile, I’d notice. But most self absorbed females would not. And I’m not gender bashing here, it’s just the truth from what I’ve observed.