Have you lost weight since going to a vegan diet? That always helps.
I've always had decent endurance, so no real comparison. I remember reading once about vegetarian cyclists, and someone commented that if you look at the diet of top riders, its almost a vegetarian diet. Eating a lot of meat is tough to digest, the grains/veggies of a vegetarian diet are easier and more nutritious for an athlete. I became one for ethical reasons, but had already been decreasing the amount of meat I was consuming.