HR lag is always an issue in interval training. In your case since the lag is long and the difference in pace 7:15 to 8:30 is large I would recommend doing these at 8:30 for the first session and see how you recover. If you feel good the next day or by two days after then do the next session at 8:15 and continue that way. This will be better than to do the first one a 7:15 and find that you are wrecked for 4 days after or unable to complete the workout at that pace.
Sneak up in the intensity gradually.