If you want to make a substantial difference in your appearance without adding very much weight at all, try working on your shoulders. Shoulder presses and lateral raises work well. If you're a runner, it's very possible you're slow-twitch dominant, so try 1-3 sets of 10-20 reps 1-2 times a week. Maybe throw in a few sets of pull-ups for balance. Even fairly thin guys look sturdy and athletic if they have some shoulder musculature. And the small amount of additional muscle mass will hardly be a millstone around your neck.
My weigh has varied considerably in my adult life. At 5'10", I've gone from 129 lb to a lean 170 lb. In my personal experience and observation, there's truly a lot of diversity of preferences among women. Sure, some body types, particularly the classic mesomorph who is lean and muscular (but not to an absurd degree), have more purchase than others. However, have a distance runner's physique will certainly not sink your chances with a significant portion of the female population.
And how you dress makes a difference. I'm not saying you need to become obsessed with expensive or trendy clothes, or spend a huge amount of time thinking about your "image." I just mean that you should pay attention to picking clothes that have a good fit for you as an individual, because, believe it or not, that can make a big difference.