Apple Watch SE not recording movement or distance correctly

My new apple watch keeps recording me as having run less far than I have. At my local track it keeps recording a lap (400m) as being around 300m which is clearly wrong. This also means my local run routes are showing as much shorter and therefore my speed as much slower. I'm normally around 6 minutes per Km but it's showing me as 8mins/km!! The watch is as good as useless if I can't sort this because I bought it specifically so I could use it to train.

Is there anything I can do apart from go through the lengthy process of sending it back and ordering a new one or having this one fixed? Or is it just that the SE has a rubbish GPS signal?

Also on the stand function I can fully walk around for 10 minutes and sometimes the watch doesn't register me as having moved so will not mark it as an hour where I have stood (less important but still annoying and possibly related).