The time delta will also surely depend on the time you are taking for the 21k though. 5mins at 2+hrs is a little over +4%. The same for someone running 80mins is +6.25%. Put another way, 5mins is 14s/km. If you’re open half is 3.46 (80min pace) then 4:00 pace is a lot slower in terms of how it would feel. It’s a much smaller relative decrease in pace at a slower starting point, and so will feel a lot closer to max effort.
For ref, I’ve not yet run within 5mins. It was only in Elsinore last year that I finally broke 90 on a HIM run, yet I’d gone under 80 open two years running. I finally got within 6.5mins when running 86 at Nice.
As a guide, my coach has always loosely set me the aim of running my open marathon pace at the end of a 70.3, whatever that is for you. But I’ve not yet done it.
I think eJC means why do you think you undercooked the bike. The TSS isn’t necessarily the best measure as this is reliant on an up to date FTP and proper calibration, etc. There’s also the issue that people use 20min power or 95% of it for their FTP, which is not FTP and therefore over estimates their actual FTP. I think the ramp test also over estimates it, Conconi’s test whilst it may have its place I’m not sure estimating FTP is it. BTW, what was your HR during that test, that would be a more accurate estimate for your threshold HR compared to the Oly bike.
In @NickBerry’s post there’s a link to a Gordo Byrn page, that is well worth a read although, like the link itself, maybe doesn’t address your question directly.
As eJC states it is usually better to see what you can actually do when you put the S/B/R together, not neessarily all three rather than individually. Obviously that is still useful for quantifying gaps and progression.
I personally don’t bother with zones in general TBH (as in 5 zones, 7 sones or whatever), I know my Aerobic Threshold (AeT) HR and make sure I am around that (usually just below) if that is the session. Otherwise I don’t set my workout by it. I monitor and make sure it is in the ‘normal’ area for my RPE and can usually gues within a couple of beats. It’s when tht is far enough off that I think something is going on that shouldn’t be. I set bike workouts to power based on either VO2max or FTP. Run is similar it’s all steady around AeT (based on RPE/actual breathing) or drifts up on hills (West Yorkshire) or specific based on VO2 max or race specific dependent on the race itself. During a race I only look at HR on long stuff to make sure I am not over cooking it because I am feeling good.
(ETA: obvs I am not a coach and wouldn’t pretend to be or a great triathlete but I do get exercise physiology and I am answering in that way.)
No I think it’s good, I’ve been consistent this year and realised a steady benefit plus a bit more after a bike focussed block.
I did the same course in June and felt like I had given my all, then fell apart on the run as expected - losing it at about 11km then stagggering from 17km
This time I’m 30W stronger on the bike but held the same power, and it felt that way. Pushing but not hard, I felt I’d left a bit on the table and ran without multiple breaks. Felt a bit of fatigue as I hit the steepest section for the second time with 3km to go but I wasn’t dead on my feet. It’s seems a bit crazy I was only ten minutes faster to be honest.
Here was the recent 70.3 bike, I don’t think anything looks weird except the max power which you can ignore, I still get the occasional power reading outage that suddenly “catches up” with a mega reading.
The June one on the right was flatter and overall 160m gain, I notice I went pretty hard in kms 2 and 3 which isnt ideal then you see the collapse at 12. I was taking regular gels and water as well.
Sept, left, hillier and overall +226m, but I seem to have paced it steadier.
Yep those were peak HR, 168, steady decline from there to 124, but i ‘deliberately’ overcooked the bike so no surprise there. And elevation gain may only have been 80m in fact, according to watch rather than phone.
In contrast, there was no steady decline in the Sept half.
sadly that doesn’t give me much…each step should have as much data recorded as possible (duration, HR, pace, power, cadence etc - ditto for bikes…a lactate sample on each step makes the data much more informative)
. Not sure whether you feel your question was answered or the TT massive have been of any help on this 
