So here it is.
Fastest 10km you can do, but keeping your HR under the 75% range as defined by the Karvonen formula
The idea is to keep the HR steady after the initial rise.
Peeking over the ceiling for short periods is allowed, but the aim to keep just under the max level.
Funkster - RHR 39 / MHR estimated 170
9th Jan
44:32
Average 126bpm / Max 135bpm
This is pretty much what I’ve been doing the last 2 months. RHR is 45 ish awake. My MAF HR is 138 ( 180 - age + 5 for good behaviour)
My best this winter is 57.40 ish at 132bpm but I’ll do another over the weekend and submit. 
It’s been years since I looked at the Karvonen, or was it the modified Karvonen method.
Isn’t HRmax by age?
Shamelessly copy/pasted from the net:
For example, a 50-year-old with a resting heart rate of 65 would calculate as follows:
So I’m 48. Resting HR 45 ( assuming this is awake)
Max = 172
RHR = 127
75% range = 140bpm.
Pretty much ties in with MAF calc.
Do you just ignore your actual max HR then? And just take the age derived one?
Like the idea of this.
Will check my resting heart rate tomorrow am
True
52 and saw 183 doing random bike intervals last week, they were hard but not maximal in a freezing cold garage …it’s almost certainly higher for a planned max hr session.
I’ll be keeping an eye on this thread
I would say a real world MHR is a better indicator.
Also gives you a higher range if you know it’s higher than the estimate.
RHR is 48bpm normally.
But 52bpm recently, mainly due to weight 
My maxHR is ridiculously high 
Like 200 
75% Karvonen is 163bpm for me, which is between 10km and half a marathon effort
RHR=40 & MHR=179 so I’ll be looking at a ceiling tomorrow of 144.
It’s going to be a long lunch break.
I think my TR has a few erroneous values. I’d say my max HR last year was 200 - though I’m bit unsure whether or not it would be that high now I’m unfit and a tad out of shape. My max hr always tends to drop.
RHR is 43.
Is that right, yeah?
That’s kind of the ceiling I look for when I’m going for a really steady low z2 run. So there or thereabouts.
My high hr max gives me a bit of leeway here, luckily.
220 minus would leave me down at 148 where I’d be struggling to run! 
I didn’t hit 160 when I did the New Years 5km
Okay, so
Hmm, don’t think that’ll be a problem. . Obviously not going to be fast though but then I’m not fast anyway 
Yeah - but this is where it gets weird for me.
By easy runs are 138-143bpm
Z2 would be 143-150 ish
Z3 would be 150ish - 165 ish
Z4 above 165ish
Z5 I don’t go there.
I’d aim to do a HIM after the swim at 155bpm.
I’m the same …
but
Older
Lighter
Shorter
Slower
But the hrs are virtually identical
Except max…
but my resting is slightly higher.
Zones are identical
Don’t go to 5 either.
So you’ve got a huge range above your z4 then if you’re going up over 200.
I normally class z2 as 150-170 can cruise in high 160s but have to really concentrate/go slow to keep under 160
Z3 is 171 to 182-5 depending on fitness.
Z4 low 180s to low 190s
Solo half would be 183 or so.
Mara would be 181 or so.
Exactly the same for me.
Might give this a go.
RHR 39 and Max of 193 puts me at 155 for 75%.
Took a look on TP at some easier Aerobic runs I’ve got and pulled fastest 10k and Im around 4:02 for sub-150 so imagine I’ll be around 39:00-40:00 maybe creeping towards 38 if held 152-155 solid on a suitable course:) Course would have a big impact on something like this I feel for how close to average you can hold.
I’ve got 140 to play with which was the number I picked out of thin air last autumn as a relatively comfortable HR, I’d guess I’ll be somewhere around 7:45 miling
