Sounds like a blast, let's try it.
Every Sunday I've been running round Richmond Park (this morning by some mad coincidence I ran past Mr & Mrs PSH who I'd only met properly for the first time yesterday! See previous post for who they are). It's mainly flat(ish) with one minor uphill and a corresponding downhill a couple of km later. I decided to set off and do about 2/3 of a lap as warmup (HR well within Zone 1) then at the bottom of the hill step it up a bit, and run to the bottom of the uphill on the next lap. This should give a test period of about 45 minutes. I then have a further 2/3 or so to finish off well within Zone 1 again. That should be long enough to notice a change even if I only get slightly faster, but short enough that I can manage my heart rate carefully during the period.
With only a slight adjustment to starting point due to pressing all the buttons on my watch apart from the correct one, this is what happened:
Objective, adjust effort to maintain HR from 146 bpm to 151 bpm
It's surprisingly hard to manage your HR within such a tight boundary. I found that when it crept up, reducing it was easy (slow down a bit), but the tough bit was getting it to stay high enough on descents. I pretty much sprinted down one or two small hills - and then have to decelerate rapidly at the bottom or it keeps on rising. No chance to relax! Keep checking the watch!
"Lap 2" is the experimental period
So, despite the very (very) slow ascents, and near sprinted descents, I think this counts as a success - I was only out of the target range for a short time, and I adjusted pretty quickly every time.
So, 0:46:47.57 is the first data point. I've made a note in my diary to try this again in 6 weeks, and in fact another 6 times throughout the year at roughly 6 week intervals depending what else is scheduled.