I have noticed that my heart rate is very high for the first 500 - 1000 m so I try to walk a km or so before starting my watch. That didn't seem to make any difference and I was still very slow over the first km whilst I tried hard to get my technique right. I did manage to get some speed up and felt my core was staying pretty tight without me thinking about it all the time. When I remembered to lean forward from my feet I started to feel I was walking quite well. I originally planned 7:30 / km but revised that to 7:15 then 7:00 as I realised I was going faster than expected. I pushed the last km along and managed to get the average rate down to 7:01.
My posture is at the root of my problems and I have been trying hard to stand upright and tuck things in at all times. I am even sitting more upright at work but I was starting to get a sore back by the end of the day.