Since I read this thread first time around, it’s popped up in my mind a coupla times riding the bike to work.
For the OP (sleepyRunner), here’s a quick summary of how I’d approach it:
First, I’d check out the performance points of my 5k PR and my new sub-1:10 HM PR and see what they are. I might learn a coupla things and they could serve to guide my marathon build-up. You want to get as much info as you can about the kind of runner you are...
Case 1:
If the points for the 5km are significantly better than for the HM, then I’d be a little concerned, because that could suggest that the downward points trend will continue towards the marathon.
I might see something like this: - 5km 1,000 pts … HM 950 pts …(predicted) M 900 pts
This would suggest to me that my long distance endurance is not as highly trained as my 5km performance, so in the weeks leading up to the marathon, I’d be very careful to improve endurance and fat-burning training every week. Otherwise I might myself cruising for 20 miles in the race and then bonking outta muscle glycogen. You need to work a lot on fuel-economy.
So, I’d be looking at the paces between M-12 (Mpace minus 12 secs per mile) and (more importantly in this situation) M+24 (Mpace + 24 secs per mile). I would do as much of the latter pace in a week that I could (within reason), building up to 10-milers and even 2 x 10km at this pace (it ain’t tough) as a single workout.
Example: Mpace = 6:00m/m, M-12 = 5:48m/m, and M+24 = 6:24m/m
Being me, I would correlate all this with HR so I could be sure my “engine” isn’t working at an effort I just know I cannot maintain for a marathon (ie: going in I’d be confident race pace is going to be no higher than 87-89% HRmax – see Cabral & Hadd thread for more details).
To gain marathon endurance, this (more-FT kind of runner) must expect to lose / give up some speed at 5km. In other words, his 5km performance should worsen as his marathon date approaches. Probably his 10-km will also worsen... What you lose on the swings, you gain on the roundabout.
Case 2:
If the points for the HM are better than for the 5km (and this is possible if you are more-ST – see Cabral & Hadd thread for definition), then I would ensure that I kept up the ability to run a near-PR 5km right up until nigh Marathon day.
Once again, I’d work all the paces from M-12 to M+24, but I would also add in some faster 5k-paced work on a semi-regular basis; you give up that 5km ability at your peril if you are an ST-type of runner.
Be careful of Mpace. It can be overrated. Often just a little bit slower than Mpace is better (eg: M+10 to M+24 above). Too, you want to make sure you are training at “effort” and not “pace”.
A marathon in a cool northern climate in April is not the same as a marathon in Honolulu in August. The effort is pretty much the same, but the pace is definitely not. We found this here one time; we trained for a marathon in the Mediterranean heat at HRmarathon and got one running pace, but then raced in a far northern way cooler climate and raced 12 secs/mile faster at the same HR (due simply to change of ambient temp).
The worst thing we could’ve done in training was to train at "M_pace". This would’ve been too hard due to the hot training environment and we would've rapidly overtrained.
Fortunately we trained at "M_effort" and took "Mpace/M_Effort" to be to 88-90% HRmax and were rewarded with a very successful marathon and ignored what the pace was. So, I would suggest monitoring much of your training by HR (especially in warm climate) and not focussing too much on target race pace.
The final pre-Marathon mode (aka Phase IIB) might only last 6-8 weeks (the longer period for the FT-runner who needs to gain a ton of endurance, the shorter period might be enough for the ST-runner who finds his performances improve as the distance lengthens, so it is not as difficult for him/her to switch to M-mode).
Until that time, work on improving your 10k-HM performances (and even 3-5km if you are a strong-ST type).
Good luck.