I wondered that too, having recently purchased a hrm to follow the twelve week 7hr (ish)plan. To keep my heart rate down I almost have to go at walking pace.

I am of the opinion though that the fitter you become the more power etc. you will be able to produce at the same heart rate so maybe today you might not be doing much but at the end of the twelve weeks I reckon you will see a difference in your output at the same heart rate.

But hey, I'm just guessing