That's a really good question. Hopefully someone else will chip in with some suggestions.
Here's how I see it. When I was younger, before high speed internet porn, I used to masturbate to fantasy all the time and it didn't cause any problems. I think it's important to remember that throughout human history men have been masturbating to fantasy without creating any sexual dysfunction. What did people do before television? They fantasised about having sex with the girl/boy next door while masturbating. So, fantasy in itself is not a problem. I'd be interested to hear other opinions.
I think the real problem is recreating porn scenes you've watched while masturbating. By this I mean replaying porn scenes when you're masturbating. I think this is the kind if fantasy we need to avoid.
The reason for the increase in PIED among young and old alike is because of high speed internet porn. I don't know what your habits were like, but I imagine they were similar to mine. Watching a video for a few seconds (minutes at most) before clicking to something else which seems more exciting, then clicking again to find something to finish to, but that wasn't perfect, so clicking again, and again, and again... Most men report the need to find something more 'exciting' (which usually means more disgusting) in order to get the same level of excitement. This is classic desensitisation. In the real world we don't have sex with a new person every 50 seconds. This is what porn is like.
The reason you need to stop watching all porn now is because even a simple still image might cause you to relapse.
I don't know if you have PIED, but it's still fantastic that you want to quit porn (I wish I had years ago). Please, just don't hit the tube sites. This kind of compulsive viewing will really fry your brain. I don't think fantasising while you masturbate is a huge problem. I think replaying porn scenes you've watched while you masturbate is the same as watching porn, and should be avoided.
I hope someone else can offer some advice. Good luck!