Why should you go slow? Surely that's counter intuitive?! Afterall, you want to go faster, right!?
Lets look inside the body. Within the muscles you have mitochondria. The mitochondria are the 'energy producers' within the muscles. Hard, intense training makes these 'energy producers' work better, but it keeps the amount of mitochondria the same.
Zone 1, 2 and 3 training builds MORE mitochondria, which means more to build on and strengthen in those hard sessions. This sounds great, and you'd assume that you should aim for zone 3 to get the maximum benefit, because it feelsnhard and it feels like you should make more progress from it. STOP! Caution on zone 3.
Zone 3 training crosses what is called LT1, lactate threshold 1. The lactic acid build up means it takes you much longer to recover from.
Zone 2 however allows your body is to filter out the lactic acid as you go, meaning there isn't a build up like a zone 3 session. This means your body suffers less for the same benefit.
You recover faster, meaning you can go harder in the hard sessions!
So, go slow to go fast. Aim to do 80% of your training in zone 2, the rest, feel free to go hard!
If you're pressed for time, try and make most of the session zone 2, then add in just a couple of short, hard intervals.