3km in 12:09 fits in well with 4.5km in 21-22mins. Blood lactate rises in an exponential manner (while it's not lactate itself which causes fatigue it is a decent proxy) so what feels mildly uncomfortable after 20 mins will feel like pain and suffering after 30-35 mins. 4min per km is doable for YOU up to a certain distance, then it progressively feels harder and harder until you feel like you're running in cement. Similarly just under 5min per km feels ok for longer but eventually if the pace is above your lactate threshold, which it appears to be for you, you will have to slow down. This is part of the misconception regarding LSD or jogging for aerobics as I saw above.
Yes, if you want to run for a longer time, over an hour say, you will need to run relatively slowly (below your lactate threshold) and therefore your HR will be much lower. This type of running can be done every day or more and it will build endurance and will result in WEIGHT (both muscle and fat) loss. This is part of why it gets such a bad rap in the strength training world.
A 20 min run at slightly above lactate threshold is a completely different session though. At a certain level of fitness you can't really do it every day without stagnating but there are plenty of good runners who perform similar sessions a couple of times a week (pure strength athletes is a completely different story). As your watch data shows, this type of session can be equally or more effective than HIIT in generating EPOC.
Don't get your panties in a bunch over the unfit comment, similarly understand that HIIT is not the be all and end all. What do you want out of your training?
If it's to maximise EPOC as a proxy for fat loss then you just showed that Lactate Threshold training is possibly more effective for you. If you want guidelines regarding HIIT because you enjoy it more, then it may help to think about the physiology of EPOC. Excess Post-exercise Oxygen Consumption is maximised by using anaerobic energy pathways during exercise. The more energy you produce via these pathways, the higher EPOC you will create. Therefore you need to get yourself well above your lactate threshold for as long as possible during the session. Using times that you've presented it could be via 1km repeats in 4:00 with 2 min recovery or a MAS type session such as 15s hard, 15s easy where you do 70m during the hard and 40m during the easy. You'll need to take a 2 min break every 6-8 mins for this type of session. If you want to do it without running, then there's about a million examples on this site and dare I say it, most crossfit met cons fit the bill as well.
Track your sessions and see what specific protocols are working but remember to keep progressing. Also remember that this type of training is demanding and if done every day will result in stagnation fairly quickly. Other types of sessions can help you look better, run further, get stronger etc. so really it depends on why you want to train.