Help with HR zone training
I have just completed a turbo brick session & set off on my run on zone 1 very comfortably, my heart rate remained in zone 1 (sometimes entering no zone) for 3-4 minutes then it shot up to a heart rate of 158 which is getting close to the top end of my zone 2 even though I wasn't putting anymore effort in, the terrain was the same and I as running at the same pace & really comfortable! My heart rate then went into the bottom end of my zone 3 (164) but again I wasnt changing my pace. I'm frustrated and confused. I really want to train using heart rate zones as i have heard so many good things about it and i'm training for a half ironman but I just feel like its not working.
Any advice would be appreciated. Thanks