Interesting question. I can speak from personal experience because I used the very same HH plan for my debut marathon. You'll know that it is a very straightforward plan, which is one of the things that I like about it, and I tended to add a tempo run into the midweek, but otherwise stuck very much to easy pace and "race pace" running.
Before my next marathon I joined a club and got into the habit of interval training. When it came round to marathon training again I kind of re-tweaked the plan so that there was an extra quality session, but once I was familiar with the weekly schedule I'd essentially written my own plan. So you could do something similar, i.e. stick with what you know, but tweak it in whatever way you feel is necessary to bring you on to the next level. HH is obviously very light on faster than race-pace running, so you might want to add in some tempo runs and longer intervals.
On the other hand, if you feel more comfortable following a written schedule by the book, the adaptations I've suggested would get your plan looking more like a P&D plan anyway. The key thing for me would be that whatever plan you do, it allows you to adapt reasonably comfortably from what you're doing now, in terms of how many days per week (e.g. maybe think about going from 5 running days to 6?), when key sessions are scheduled, etc. One of the best things about the P&D book is the first section before the actual plans, which explains the various elements of fitness needed (endurance, threshold, VO2 max, etc) and how to exercise them. Maybe read that and see if it gives you any pointers.
