I'll try to be quick because saying "vegetarian" makes people lose their minds.
I've been vegetarian for 20 years. I was 6'0 and 150 pounds at age 21. About 168 at age 30. About one-seventy-five (my numeral seven doesn't work on my keyboard) at age 40. I ate my share of cheese and eggs.
Then I went vegan. My weight went down to 150 after about one year. I went down slowly, so I stayed strong. My times dropped like crazy because I felt light as a feather.
The vegan diet helped me. I also kept an eye on my fat intake (stopped using so much oil in cooking mostly). The weight came off very easily, went below 150 and still felt strong.
To get the build you're looking for, I'm sure you realize you must go vegan and give up the cheese, milk, and other foods. My wife did the diet with me. She's medium to large boned--she looked fantastic on this diet--not anorexic, but thin and muscular. I had a 6-pack and did no situps.
Go for it. It will help your running. Try to stay away from the sweets too. A good, simple diet is the one thing American runners are missing--something that could really help them. I wish I would have tried it while I was young and competitive.