How to better my PB in 1.5month?

Age: 32 (33 in the end of May)
Sports background : 25 competitive years in different disciplines (Taekwondo, Swimming, Cycling, Running, Triathlon)
General achievements : 350k/day w a mtb (2005),
315k w an avg of 33km/h on a road bike under 104F weather (2012), vo2max 76.5 then.
Half Ironman finisher (8th overall, the world Champion was there and won it) in one of the toughest half ironman courses of the world. (North Cyprus, 2014)

Chronological Marathon background : 3:07, 3:02, 2:58, 2:57, 2:56, 2:54, 2:48, 2:46 (Oct, 2014 with a peak of 98km/week, 5C temperature)

Last marathon : 2:55 (1st of March, 2015, 3 days ago, with a peak of 119km/week, the race temperature was 20C so hot even though I perform good at hot temps) why I couldn't better my time?

Answer : I did something silly for an experienced endurance athlete and started with 88%maxHR and even reached up to 90-91% for a very short time and paid it off at the 14th km with a side stitch. Also tried a different fueling type (instead of taking traditional sticky gels every 40-45min, took High 5 fluid gels every 20min as it was suggested on its guide)
After the pain of side stitch, I had 2 options to compansate my race;

1) Switch to walk for 5min to lower my HR and then go on my race at my desired speed/intensity with no problem
2) Just keep running but at a lower speed/intensity till I feel better or at least to guarantee crossing the finish line.

Since first option might trigger cramps by having the side stitch at the 14th km, didn't want to take risk of having a huge cramp with higher lactate, so I took the 2nd option but no matter how I had slowed down, my HR was 170bpm till the HR monitor went wrong at 33rd km and I took the HR band/strap down on my abs, to feel relief on my chest. But according to how I felt, I bet my HR was still around 170bpm also after km33 until the finish line.

So, I got the lesson, okay, comfortably hard means 82 to 88% of maxHR and you can run marathons with this intensity range but you should build up to 88 as you cover the miles in the race, you can't start with directly 88% and expect yourself to keep running the entire course with that intensity.

Anyways, until here there was no question, I found my answer for this case and wanted to share a brief race report. Now here is my question;

This was not my target race, the big one is on the 19th of April in Germany. So I have 1.5 month to go.
Obviously I am currently in my recovery week, lets say "reverse taper".

Do you think I can run any sub 2:40, 1.5 month later with the most proper training? I think we shouldn't consider my last marathon as a reference since I made a tactical mistake by starting of agressively. I would finish it in 2:42 to 2:44 if I hadn't started that fast.

And what type of a schedule you would recommend me to do to achieve this goal?
Thanks a lot in advance.