Hi,
I usually don't respond to let's run boards but wanted to respond to yours specifically since I had similar questions about veganism before making the transition-- not to mention I'm a big vegan advocate and love helping others to live this lifestyle!
I went vegan the winter of my junior year because my fall XC season had went poorly, but a girl on my team who I had used to be running circles around (and who had recently went vegan at the time) ended up beating me consistently by the season's end. So I decided that that was the way for me to improve. I had a decent amount of natural talent (that previous track season I had run 66 for 400, 2:30 for 8, 5:51 for 16, 12:25 for 32, and 20:37 XC 5k as an omnivore) but that next season after being vegan for a few months I ran 64 for the 4, 2:24 for 8, 5:28 for 16, 11:49 for 32, and 19:49 XC 5k. Mind you this improvement was all off no additional offseason training whatsoever-- I only did swim team in the winters, with no running until about 2 weeks before the season's beginning. Because I did nothing different in my training between the seasons, I can only accredit this success and drop in times to my new diet. It made me feel fresh and allowed me to no longer feel weighed down in my training. I had better recovery time. I felt energized. Obviously not everyone will experience the same increase in physical ability as I had by going vegan or even vegetarian, but I promise you, if you give it a try you will see something positive! I did lose a little bit of weight after going vegan (from 130 to 120lbs)-- but this is natural, and will NOT be at the expense of any intentional or unhealthy method of weight loss. Make sure you eat enough to satisfy your hunger needs.
Just be sure you're doing veganism the right way (i.e. eating nutrient-rich foods, take B12 supplements.. these could even be daily multivitamins, and research how you can get all your essential nutrients through eating foods that you already like + adding some new foods such as veggie burgers/mock meats!) and you'll be fine. Doing it the wrong way can make your health worse!!
I don't run anymore but that's not because I feel 'weak' or any bogus reason like that-- I just lost interest! Unfortunately I was injured my entire senior year from stress fractures so I never got to see my continual improvements :/ My iron levels also test really high on a regular basis, so no need to worry about having low levels by going vegan; now I've been vegan for 3.5+ years and I can honestly say that it's the best decision I have ever made for myself. But of course, you do you!! You know yourself best.
Hope this helps girl!!