I don't wish to start off sounding pedantic but can you please clarify 'migraine' true migraines include vomiting, visual disturbance and often times hospitalisation and heavy medication is the only remedy. If running is causing this kind of impact I would suggest that you should see your doctor and start some serious inquiry into the situation.
If you are experiencing really awful headaches after a run AND you have cleared issues relating to your cardio vascular health etc then it is potentially a functional/structural issue. Avenues of inquiry would be 1. Breathing 2. Posture a) skeletal misalignment b) muscle function - may I suggest that you research your local area for running focused Physio Therapy, Exercise Physiology that have studied www.functionalmovement.com .
It could have a lot to do with your posture and recruitment patterns as you fatigue. The upper body - Upper Trapezius, Scalenes, SCM, Splenius group and co. have a propensity for firing up trigger points that cause horrible headaches. It is also possible that the source is further down the kinetic chain and a mis-alignment of the pelvis is creating torsion that manifests as headaches.
I think that the fact that you have been able to improve your distance through changes made to your running technique ie. posture is a good clue that there is a functional involvement. The really good news about that is that any break throughs made in that arena will have a really positive effect on your running.
Good luck and if you feel like it I'd be keen to hear updates. As an RMT with a passion for running I love to dive deep into this kind of stuff - I'll ask around my clients and see if any of them have had the same experience or know some one who has.
Sounds like you are on the right track to figuring things out! Might mention seeing an accupuncturist since you mention pressure points as possible triggers. They may be able to help identify trigger points that you might be able to use to prevent and/or treat the migraines. Your migraines sound very vascular which is common. Migraines also tend to occur post stress...We in the medical community (I am a nurse practitioner) think there may be a component of vasodilation in the vessels of the brain as a trigger, it puts a stretch on the nerves which may be the trigger for pain. When you stop running, vasodilatation occurs in the small vessels in the scalp and brain and this pressure may be triggering. Some things you can try to prevent....Make sure you go in well hydrated and drink if you need to to stay hydrated during your run. Warm up slowly and ease into your run (sounds like you already do this). Taking a medication like ibuprofen before your longer runs or hotter runs may also prevent. If you know which types of runs trigger, then pre medicate. In general exercise can reduce the frequency and severity of migraines, so keep running! It is important that you have these headaches evaluated to make sure there is not a serious cause, but it sounds like you have already done that as well. Best wishes from a fellow migraneur!