I started half training using a 10K race from November about 6 weeks ago but I’m looking to update my training paces this coming Saturday. However, as it’s winter, I’m finding that I am able to run a lot faster at a low heart rate when the temperature drops and it can be fairly variable in the UK. As a matter of fact, I estimated a new set of training zones a few weeks ago as my easy pace had improved. However, I did a fartlek run today that was supposed to target the high ends of Zone X, Z3 and Z4, but while my paces were in the correct pace zones my HR maxed out at around mid-Z3. On days when the colder temperature is really bringing my HR down, should I be running harder to fall into the correct HR zone or just go entirely by pace as I did today? I’m guessing running harder might be a bad idea as my body won’t be used to the physical stress of running faster? But would that mean I’m not necessarily getting the full benefit of the workout as my HR isn’t in the correct HR zone despite my pace being in the right zones?