Here's a really valuable piece of information that wearing an HRM can tell you (and all the folks from out online running group woulda spotted this quickly). When your HR does not come up to expected levels (and with experience you quickly learn what the HR should be at any given pace), this is a huge sign that your are not fully ready for this session.
There could be a number of reasons for this: the most likely might be not enough muscle glycogen, or your muscles are not sufficiently recovered from a recent session. Whatever the reason, if your HR doesn't come up to expected levels (and you'll usually find the session feels "harder" than usual) then it's a sign you should not be doing that run. (Don't fall into the rut of "it's Tues, it must be track".)
It's tempting to see a lower HR than usual and be tempted to believe that you've broken through some kind of training threshold and now need a lower blood supply at the same pace ... but the truth is, it's more likely to be low muscle glycogen or insufficient recovery from a recent session (even lack of sleep in recent days from something going on in your lifestyle). Just take 3-4 days of easy running combined with adequate eating and rest and run the session again; probably the HR will be exactly where you expect and the session will feel MUCH easier and you will have greater confidence that you could go further at the same HR, but you don't.
You didn't want to run faster ... another clue for low muscle glycogen.
With an HRmax of 185, your HRmarathon zone is gonna be 160-165 (not above, not below). You'll probably average 162-163 over the whole race.
I don't like to see folks aiming for a marathon unless their shorter-distance performances predict that their marathon target is possible. For a sub-3hr marathon, that would mean a sub-19:00 5k (and 18:45 is even better, especially for males). If you can't do that, I would recommend concentrating on improving the 5-10k times before looking to the marathon.
Some folks can run good marathons straight off Phase I, but that's usually because they come into it off some interval-trained background and thus the aerobic conditioning was their weakness and Phase I fixes that up real good. So they've often got decent performances at 5k, but drop off drastically when they go longer.
If you have never broken 19:15 for 5k, don't be aiming for sub-3hr marathon till you correct that.
I see that you are already training at HRmarathon zone (160-165) but only started at 150HR and began pushing it up. With a 185 HRmax, I hope you are doing a lot of miles at 130-135 HR. The faster you get at 135 HR, the quicker you'll run at 165 HR.
See Joe's numbers:
Dates — 140 — 150 — 160 — 170 — 180
11May – 7.56 – 7.22 – 6.42 – 6.05 – 5.40
06Jun – 8.03 – 7.17 – 6.36 – 6.01 – 5.33
29Jun – 7.23 – 6.49 – 6.12 – 5.42 – 5.18
04Aug – 7.18 – 6.36 – 6.00 – 5.33 – 5.10
5:10m/m at 180HR was never gonna happen as long as 140HR only generated 7:56. But once 140HR began to result in 7:18m/m pace, then 5:10m/m at 180 because do-able. It's the total toothpaste analogy; you only get the maximum possible if you begin squeezing from the very bottom of the tube. Starting halfway don't cut it.
