Intervals probably shouldn't be your main focus in marathon training, as endurance is much more important than speed.
Lots of people have had very good results from following P&D, but it won't suit everyone. I think the sessions get longer at the same speed, so the first interval session should always be a bit 'easier'. Although I'm not sure what P&D plan you are following as I'm doing the 18 week/ 55 mile plan and just starting intervals with 8 weeks to go?
I also agree with andrews148 comment - if your 5k speed is 5min/mile then you should easily be sub 3 hour rather than 3:10(ish) for the marathon.
A heart rate monitor isn't essential, so I wouldn't worry about not having one. As you've run a few 5ks, you should be able to use plans based on your race results to calculate the training paces for a 10k. Have a look at the smart coach on this web site for an example of this.
Don't get me wrong - I have a Garmin GPS watch with HRM and I really like it, and I find the HR reading very useful (mostly on slower runs to make sure I don't go too fast). But it is a luxury. When I started I just used a stopwatch to record how long I'd been running and this was just as useful for keeping track of my improving times.
You should see improvements in your times if you can increase your average to 30 to 35 miles per week. The normally quoted rule of thumb is to increase by a max of 10% per week. As you've been running a relatively short time, I wouldn't get too worried about specific speed sessions and target zones etc, just think about running one or two sessions per week 'comfortably' hard and then making sure the other sessions are run at an easy pace. Its a common mistake for newcomers to think all sessions should be tiring, whereas some of the most beneficial ones (especially for HM up) are the slower ones that help build your base endurance
For you, you would probably want to start by increasing your shortish runs from 3 - 4 miles to 5 - 6 miles. When you've done this, you could add a fifth session into the week, and drop the mileage on the shorter runs so that total mileage per week stays within the 10% rule.
There's no excuses with this one as the course is as flat as you can get for 26.2 miles.
If you can run a half in 1'45" then you should be able to achieve a sub 4'00" full marathon. However, you do need to build the endurance to do this.
I ran this one three years ago, and my preparation was the Cardiff HM in the October before, and the Llanelli HM (on the same course as the marathon) in the March. Maybe you could follow a similar plan - training for Cardiff (or similar) in a couple of months, then using this result as a baseline for your marathon training, with the Llanelli HM as a final preparation/test prior to the 'big one'?
I'm following the Pfitzinger & Douglas plan from their "Advanced Marathoning" that is definitely worth reading for the explanations of the various training that contribute to running a marathon. Even if you don't follow the plans, it will explain the benefit of different types of runs etc.
Based on this book, and your schedule, I'd suggest:
Long Run on Sunday: aim to complete 5 runs totalling 100 miles prior to your three week taper. This could be 2x 18 miles, 1x 20m and 2x 22m (very little benefit going over 22). Ideally, if you've gone over 18 miles on two consecutive Sundays, cut back down to 12 to 15 miles on the next Sunday (I think you've got enough time for this - but might have miscalculated)
Mid-week longer run: try and get this up to 12 miles, building gradually over a few weeks.
For the Sunday run and mid-week run, start slow but gradually build speed until you are finishing at roughly 10% slower than target marathon pace.
Intervals aren't the best "speed" session for marathon training, try swapping your Tuesday session for a tempo run. Warm up for two miles, then run between four and seven miles at half marathon pace, then finish with a two mile cool down.