HR and AT Training

Question for the runners out there -

I completed my first IM last year, which was trained for in a completely unsceintific fashion; no training in HR zones etc. This year I am planning to race IM again and need to know how I should structure my AT runs. Currently putting base in and therefore doing all runs below threshold, but should I start with a low HR and work towards threshold at the end of the run session, or get the HR as close to threshold early and then try and maintain it?

The latter doesn’t seem to work for me. It goes high early and keeps going!

Thanks.