If you have a high aerobic capacity then you can and should mix in more high intensity to your plan. Where you do this in the Transition period depends on how worked you are from your last major effort (expedition, race, etc) You can’t go wrong by being a bit conservative and focusing on the aerobic base in these early week. To keep some intensity in your program during this time I’d recommend using short speed or power workouts like hill sprints. These will provide a good base for more extensive HIIT later.
I prefer do the strength/speed/HIIT workouts on the same day when doing 2-a-days. Another good option is to do these higher intensity workouts early when you are fresh and then the low intensity aerobic work later.