Cardio fitness is a strong, consistent predictor of morbidity and mortality

That's really the opposite of what sports are supposed to be for 99.9% of the people. Who cares how life and training of some fastest guy in XYZ category looks like? Go at your pace, progress slowly also at your pace, also good strategy for life overall. You also don't take tips for driving in traffic from top formula 1 driver, do you, sorta worse than useless.

I do naturally push myself a bit when I run alone, to the point when even say 5-6km run makes me tired and drenched in sweat, but certainly not first 10-20 runs after some lull (like now broken foot, starting gently next week). And running is really just training for other more adrenaline sports for me, not chasing kms and preparing for races. But never pushing beyond whats OK for my body, and with experience one knows oneself very well in this regard.

That's unfortunately not fully how it works. For maximum benefits, you would want to train your aerobic energy system, and to do that without risking injury, zone 2 is the recommended zone. The claim is that pain is gain, but more often than not, pain is your body telling you you are over the limit. By the description of your running experience, you were possibly closer to zone 4.

Of course, the body gets lazy, so you will want to vary your training. Repeat the same type of load, and your body will get efficient operating at that load, meaning your fitness gains will eventually stagnate. That's where the other zones come in.

At the zone 3/4 threshold you start using your anaerobic energy system more, with a stronger build-up of lactic acid as a by-product. Training in that range means your body will get more efficient at faster-paced work. Of course, as you build-up lactic acid, it also means that it is not a pace you can sustain.

But if you do short bursts of exercise at the lower-end of the anaerobic zone, you are actually contributing to making you aerobic energy system more efficient, without over-straining your body too much. And it's exactly the type of exercise variation your body needs to not get into the "lazy" mode.

The traditional way of approaching that is to have a base of zone 2 days (say 80% of your runs), and a few days of zone 3/low 4 days (say 20% of your runs), so that you alternate your loads.

Another approach with similar benefits, especially if you train less often, is the Fartlek approach, developed in Sweden in the 1930s. That's a type of training where you mix-up the load intensity during your runs, in about the same proportions. Fartlek means "speed-play", and it describes the method well. It's generally about approaching your training as you feel it that day. At some point during your run you might decided to increase the pace for a short time. Or you see a small hill that you can decide to attack at a faster pace, thereby changing your training zone. It's all about having fun and making your trainings less boring.

There was this belief in some circles until not that long ago that if you would enter a zone 3 or zone 4 training load during a zone 2 training day, you would mostly destroy the benefits of a "pure" zone 2 training. (Or the other way around, having some zone 2 stretches on zone 3/4 days). That turned out to be nonsense.

All in all, for good progress, and against boredom, Fartlek is the way to go, all the while keeping the bulk of your training in zone 2.

> For every 1-MET (3.5 mL/kg/min) higher level of CRF, we identified substantial reductions in the risk of all-cause, CVD and cancer mortality. We also identified significant reductions in the risk of incident hypertension, heart failure, stroke, atrial fibrillation and type 2 diabetes per higher MET.

> For most, a 1-MET higher level of CRF is attainable through a regular aerobic exercise programme. For example, in a large population-based observational study of over 90 000 participants, nearly 30% were able to increase their CRF by 1-MET (median follow-up was 6.3 years) without intervention.