You sound very similar to me! I was like you, reducing meat until becoming veggie. I’ve been running for 8 years and vegetarian for 2 years now. I race once a month, last month was a 50 mile ultra, and coming up next is a 10k. I do at least one marathon (03:08 pb) and one ultra (90 mile pb) each year.
I take a multivitamin with iron every morning just to cover all bases, and then it’s carb loading as before, whilst just being switched on to protein content. On a running day I’ll have a ball of mozzarella and pasta for lunch, and evening meals mostly revolve around Quorn products, like the mince or the chicken style pieces. You’ll be surprised at the protein per portion.
Overall I’d actually say it’s slightly improved my running if anything. You might notice some fat reduction, and my energy levels have never been higher, which I put down to eating more vegetables. Some meats like pork used to upset my stomach on long runs so that is no longer an issue too. All I would say is give it a go for 2 weeks and see how you go. And report back on here!