I do like that! My bike has gears 1-8, I found that on 3 I can go with a comfortable resistance, not too easy not too hard, for more than one hour. When I do this I measure my heartrate by simply putting my fingers to my neck, and check my pulse for 15 seconds, then I multiply with 4. For me at this level my heartrate is 33X4=132. I'm 30 years so my maximum heartrate is 220 bpm - 30 = 190. Which means that 132 is 69% for me.
So when I do HIIT, I follow johns plan, and to increase my heartrate I pedal at the same speed, but I increase the level of intensity. I go from 3 to 4, then to 5, to 6, and then back to 3. When I go on the higher gears, I try to maintain the same speed and lately as my form is getting better I even go a little faster.
Somedays when I feel more tired I just try to keep the same pace. And when I measure my pulse it's getting much higher so I guess this is one way to do it.
Basically you can feel your way to what is appropriate to you and how it works on your bike, don't worry you will work it out! Good luck!