if you do the session on the treadmill, then set the incline between 3 to 5 and then do a rolling hills session, so 5 to 8 minutes at the incline, after a 10 to 15 minute warm up with the incline at 1 ..
after the 5 to 8 minutes at incline, lower to 1 and run 5 minutes at that level
then increase back to 3 to 5 and run 5 to 8 minutes at that incline
do this 4 to 5 times according to how you feel
ps.. if we working off heart rate (on average) 75% effort would be around 160bpm .. the effort would be personal but it is also not meant to be at an effort which is not too hard