help! with HIIT training

[tskfromnzl]

im a bit concerned about high intensity interval training that i have started to do, is this the right way to do it

on a treadmill, i do:

5 minutes warm up at 8km/h
then i do 20-25 minutes of HIIT training like this

1 min 6km/h
1 min 12km/h
1 min 6km/h
1 min 12 km/h
1 min 6km/h
then higher intensity to
1 min 12km/h
1 min 8km/h
1 min 12km/h
1 min 8km/h
1 min 12km/h
1 min 8km/h
1 min 12.5km/h
1 min 8km/h
1 min 12.5km/h
1 min 8km/h
then for the final five minutes i do
1 min 13km/h
1 min 8km/h
1 min 13.5km/h
1 min 8km/h
1 min 16km/h
then
5 minute cooldown

the problem im having is that i should be feeling more tired than a regular 25 minute cardio session(i do 4km in 25 minutes during these), however after i do these i feel completely knackered as compared to the HIIT.

why am i more tired in the regular cardio than the HIIT? am i doing it wrong?