Thanks for replying Gavin.
Temperatures over here are below 0 at the moment(the runs above and runs on days 5 & 6 were ran at around -2 C) and I'm not sure if it's a good idea to force my throat(ie gasping) on a 4k TT at least for a week or 2 untill temperatures are around 4-5 degrees C.
I see a few options at the moment. What do you think?
1. do a 4k TT anyway and start from there
2. continue with the build up for a week or two untill temperatures are above 0
3. ditch the build up, do some easy confortable runs untill temperatures are above 0 or even some strength training
Later EDIT: looks like the weather forecast for middle to end of next says it should be getting warmer. I'll try to do a 4k TT then.