> Increased strength is adversarial to improved cardio efficiency.
This is absolutely untrue. If you take a runner who doesn't squat and get them to squat (and get stronger) they will become a better runner. By becoming stronger, each stride becomes more submaximal. Of course how strong they can become will be modulated by how much weight they can gain--in the case of a runner, that will probably be a very modest amount before the increase in weight begins to negatively impact their performance. But even with extremely minimal weight gain, they can become significantly stronger from an untrained state in a way that only positively impacts their running performance.
> The adaptations come from the cardiac system. Basically, the body can use more oxygen more efficiently.
You misunderstood my question. If the session is easy, what is the stress which sufficiently disrupts homeostasis to lead to an adaptation? Easy things do not result in adaptations.
> Someone who can lift 440lbs but who sweats when they eat isn't fit, either.
Sure, they should do their cardio.
