Chimpster: I think my MHR is 185bpm because I measured it with a heart rate monitor. I'll try the 5x2 mins at 10kph; that seems to make sense really.
The Garmin training plan tells me that a long run should be 65-75% of MHR, threshold run should be 80-85% of MHR, etc, but on the Garmin Connect site which stores all my activity data, there's a table with a column for bpm, and a column for %HRR. I was assuming that 122-135 bpm = 50-60 %HRR meant that I should set my watch to ensure my heart beats 122-135bpm if I want to run at 50-60 percent of my MHR. Perhaps I've misunderstood something here!
I'm using a Garmin Forerunner 405CX watch and following Garmin's Training Plan - Level 2.
Today's Week 2 run was to be 5x2 mins at 10k effort with 60 secs jog between efforts. I understand 10k effort to mean 92-94% of my Maximum Heart Rate (is that the same as Heart Rate Reserve?) of 185bpm, but I could barely reach 170bpm and had trouble sustaining that for 30 seconds, let alone 2 mins!
I'm assuming that I'm doing something wrong here. And with my troubled relationship with science and numbers, that doesn't entirely surprise me.
Sheddy and Sazza, I've taken your general advice about slowing down - Monday and yesterday I ran two miles, each mile lasting 13-15 minutes instead of 9.5 minutes. Yesterday I ran a total of 30 minutes with a 2.5 min break in the middle, with some hilly terrain.