It's interesting to finally get to know what the underlying issue was that motivated you to come here for advice in the first place ....
The only reason I even wanted to set up the apple mail is that we could not figure out how to attach a photo or document to a gmail or yahoo mail account using the mac.
Ok, this is my advice on what I've learnt from you as to what your needs are and what I think would work best for you ....
I think you should have a Comcast email address, the one Comcast would have given you when you joined up ... and you mentioned having one but not using it ... you will actually need this for online shopping etc where they don't accept free email account addresses for security reasons.
I have my ISP given email address and a few Hotmail addresses and also a Gmail one too ... I never give my genuine ISP email address out to just anyone. If I'm unsure about giving out my real email address to someone, I feel it's best to give either a Gmail or Hotmail address.
So, you should consider looking at it like this; having a secure email account and then the other Server account/s that you use when you're unsure.
I would give them
[Comcast] a ring and see if they will help you set up the account/s in Mail.app ... it's not hard but if you haven't done this before it may be confusing, I know how I felt the first time I needed to configure Mail.app.
I suggest also that you create IMAP accounts (IMAP refers to the *type* of account you are using). IMAP keeps a copy of your mail on the Comcast Server whereas POP mail accounts download the message from the server to your Mac. The reason I prefer IMAP over POP3 is that if something goes wrong on your MAC and your email accounts are POP3 based, then it's possible to lose all your mail .... it's happened to me and it sucks! So having the original mail stored on the Server is good ~!
Regarding Gmail ... it's possible to retrieve your G-email using Mail.app and here's a how-to link:
Enable POP in your Gmail account.
So I suggest you ring up Comcast and get them to help you set up some IMAP accounts and if they won't then I will. When that's done we'll tackle the Gmail issue if it's still a problem ....
Finally, don't think yourself clueless cos you're not ....
.