Thanks karliejaye. I actually was asking the question in the context of building and keeping muscle. I've noticed the past few years that I rapidly lose any muscle I manage to put on. I spent all winter building up my legs with all sorts of torture exercise, no stirrups, jockey stirrups, etc to the point where I had rock hard legs. Two months later after some lighter riding and it's all gone.
I'm not worried about bulking up at all. The dangers of that are a good 10 years past me. I'll be happy with enough muscle tone to sit the trot and jump safely.
I did buy that whey protein stuff. It's a little wonky tasting, so once that's gone, I'll probably try your stuff and see if it goes down easier.