uncle fester!

I agree, good core work is the answer, or you could wear a large pointed hat, then your shadow will look entirely different

It is not just core strength - you can have all the core strength you want but this will not magically transform your running form. You need to make a conscious effort to train yourself to maintain form ie if you are getting tired concentrate on form and correct it and keep doing so until it becomes automatic. Having decent core strength merely enables you to do this but is not a magic bullet.
The last 2 episodes of TalkUltra in the training talk section address this. It is not just a matter of just core strength but how your feet contact the ground, cadence etc
Or just get a pointy hat as EKGO says

So a new hat is now top on my xmas list, and i'll be singing the mantra 'shoulders down' as I get tired...

thank you