Some long-time vegans have trouble eating meat again becasue they are used to not producing proteases, the enzymes needed to break down meat. If you can already digest fish meat with no problems I don't see why you would have issues digesting red meat. The main difference between "white" and red meat would be their density and presence of myoglobin, which should be taken care of the stomach's acid and the proteases.
Btw congrats for giving the PB the benefit of the doubt. I wish other vegetarians did the same.