You say your goal is "losing weight".
How do you lose weight?
Daily calorie deficit.
Two parts of that equation. Calories consumed. Calories burned.
I'm leading you here. The calories burned are what matters
if all you are concerned about is losing weight. Therefore, in your example you probably are burning almost the same amount of calories. Working hard in 20 minutes, compared to working light in 40 minutes will yield a negligible difference.
The reason I put emphasis on "if all you are concerned about is losing weight" is because there are far more benefits to cardio than just excess calories burned. Those other benefits are where you start considering elements such as intensity, length, intervals etc. But if weight loss is your only care, then the amount (and not the how) of calories burned is all that matters.
If you want to work
extremely light for 80 minutes, you could still burn the same calories your 20 minutes hard did. The extremely light work certainly won't challenge and improve your heart, and as such isn't likely to affect your endurance. But you'd burn the same calories.
That's why even if calories burned are your only care, most people still choose moderate-hard cardio. It means you can more efficiently use your time. I'd rather spend 30 minutes working hard, than a whole hour working light-- to get the same calories burned. I don't know about you, but most people don't have excess time to waste in a day. So again, if your only concern is calories burned (and if you are just looking to "lose weight", then it is) why take twice as long to get the same results (calories burned)? Furthermore, if you can work hard in 20 minutes, why not push that hard work to 40 minutes and DOUBLE your calories burned?
