Thank you for the reply. I've given it a read and based on the information I've read I think it's ok to drop mileage while adding the tempo/threshold run.
Is my understanding correct that during this phase one of three workout types can be used on the tempo days? Those workouts being either a 20 minute tempo, a 40-60 minute steady state (slightly slower than tempo), or a tempo interval (slightly faster pace than tempo), set of about 30 minutes?