Hi, excellent question.
I ran 4:06 in college and post-college transitioned to veganism over the course of a couple of months. It's been 3 years and I've maintained the diet. I'll just make note of a few things I wish I knew earlier.
- Pound that B12! You just can't get enough of it in a vegan diet unless you eat pounds of mushrooms every day. Take a regular supplement or season food with nutritional yeast. I take iron as well to be safe, though you can get that from plenty of plant-based foods too (legumes, grains, nuts, etc.). No issues with protein. Unless you eat just carrots 24/7, you won't have any issues with protein. That's a myth.
- Your body does need saturated fat to thrive on higher intensity training. Enjoy processed foods occasionally; you need the fat, omegas, carbs, and obviously calories. I tried to go entirely just straight-up plants for a couple of months and found it really difficult. Too much fiber, not enough carbohydrates.
- Your body will recover quicker if you treat it right. Plant-based foods don't have nearly as many inflammatory ingredients in them so you bounce back quicker. Just make sure you get a good amount of protein/carbs in your system quickly after a run and especially a hard effort.
- My body didn't actually change a ton in the long run but it did initially. I lost weight but that was because I just wasn't doing it right and being stupid by combining the vegan diet with counting calories (all while training Spartan-style on 60-80mpw with intense workouts). Dumb. 10/10 would not recommend. After I nailed the ins and outs of the diet and found what worked the best for me, my body plateaued out and I was just more lean and stronger than before.
All in all, if you do it right, I think it's a good thing. It certainly was for me. Ended up running 4:01 and 14:07 and still going strong.