That’s a very good question. In short, I was almost in the same shape I’m in now. The long story is that when I became vegan I had 5 failed operations and I had to stop going to the gym and I didn’t watch my diet for several months, so I gained weight and lost some muscle, and recently I managed to cut down to my old shape. Now, I’ve just started to bulk up and will continue to do so for a year or so, the plan is to see if i can build muscle on a vegan diet. It will not be easy, especially that I’m 37 years old now but we’ll see!
Believe it or not I don’t do cardio. I walk a lot during the day, on average 12000 steps, and i sometimes llay tennis but that’s it, and I lift weights 4-5 days a week. See my reply to one of the other comments for more details.