Health care proxy

I just got off the phone with my dad a little while ago. He wants me to be his health care proxy (he is going to have an operation to remove cancer from his lung). Obviously I will be researching the details of being a health care proxy (I get the general idea-make medical decisions for him if he is unable to) but has anyone else done this? Could you tell me a little bit about what it was like for you? Any advice for me? The problem is that we don’t live in the same state so I’m thinking that might be a bit of an issue. Thanks in advance.