I have done a few. Not too much different than a tri. The main difference for me is that I normally do not warm up that much for a triathlon. Just a quick swim. For a duathlon you need to warm up more, for obvious reasons. They are fun, I am sure you will have a good time. It seems so streamlined after setting up transition for a tri.
Sorry you can't swim anymore. I would be rocked by that news too.