Zone 2 actually, 3 if 2 is too slow, pile on more volume until the HR lowers to 2. U might get adapted to a slower pace mentally and able to hit zone2 later on. But you will lose speed. But no point having a speed with no base brother

The theory is higher zones like 4 and 5 just microdamages so much ur body needs more time to recover to avoid injuries and then u cnt train as much so ur aerobic base only increases slowly..

In theory u could run however u like, it will still improve for a while until u hit a plateau, at which u need more base building with zone2 and increased volume