Interval training 400m x 10

Today's training is try to emulate the run/walk sequence that I am planning to use during Army Half Marathon. I am planning to do a 1:1 ratio by time, 6 min pace running, 8 min pace walking. So, the average pace will be 7 min and I will finish the 21km at 147mins (2 hr 27 mins).



I ran too fast in my first 5 intervals and drive my heart rate to 165+. My last 5 running get to the right pace but my walking is too slow. Overall average is 7:06 per km. But I need to control my initial pace to avoid burnout and need to do it three times longer in distance. I need to get to the correct target speed.