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" Please can some experienced types give me some calm reassurance? Advice? Thank you!"
Good luck for the HM. I was given some great advice for my HM. Start off really slowly, right at the back, then gradually you can find a pace you are comfortable with. If there are pacemakers running you could position yourself behind one to ensure you keep at an even pace.
I was aiming (in an ideal world)for sub 2:30 and kept with the 2:25 pacer all the way. Then at the end got ahead of her a little to knock a couple of minutes off. For your first HM your main goal is to finish so don't worry about time too much.
So many people head off too fast and cant keep it up and you'll end up overtaking loads of people later on who are struggling.
Finishing is a fantastic feeling though. I started crying the last mile i felt so emotional when I knew i would finish. You will never replicate that feeling at your next one (that one will be more about time).
Good luck