It sounds like you and your dad have a good relationship, and if your mother wasn't there it's understandable that the bond has been very important to you.

I think the suggestion of talking to your dad and making sure you say the things you mean to him is an excellent one. Although hopefully you will have many years together it's never too early to say the important stuff.

My husband is older than me and the prospect of losing him has been an issue for me, the fact is I'm likely to have a lengthier widowhood than most. I have at times fretted over this, because I love him dearly, but I know I will have to face life without him at some stage in all likelihood.

Here's a couple of things I've done to prepare/cope:

I tell him the important stuff, about how much I love him now - I tell him I love him every day. I do not want to have regrets.

Over the years I have built and developed other important friendships and relationships in my life. It's a tough call to be 'everything' in a persons life, and not fair on them - me building other relationships has taken the pressure off him somewhat. I'd encourage you to also build and develop other important bonds in your life too, those people will be both a support and a reason to go on should you face the loss of your dad.