20 Minutes hard or 40 minutes Moderate

The one that burns more calories, but..........

fat loss comes from a caloric deficit (ie what you eat) exercise my help with that deficit but should be thought of in the context of health and fitness. Exercise presents a bit of a paradox in terms of weight loss.........you need to eat more to fuel the workouts which negates (at least in part) the additional caloric expenditure.

The one that burns more calories, but..........

fat loss comes from a caloric deficit (ie what you eat) exercise my help with that deficit but should be thought of in the context of health and fitness. Exercise presents a bit of a paradox in terms of weight loss.........you need to eat more to fuel the workouts which negates (at least in part) the additional caloric expenditure.

Someone once told me though that fast workouts burn carbohydrates whereas slower training burns fat. Any truth in this? Sorry OP, hope I'm not butting in..

I would advise you to go 40 min moderate. This is strictly from personal experience.

A lot of times I've seen and even experienced that people tend to go hard for a short period of time, think they've done a lot because , well they've been sweating a lot, give up, and go home. I would advise the longer workout as, not only will you get a calorie burn that you want, but it will help you gain stamina of not only running but being at the gym and working out.

So, I don't think you should kill yourself for 20 minutes, take it at a moderate for 40 and I think you'll see more of a gain both physically and psychologically.

Anyways that;s just my thinking!

Do both. Variety is key when planing out your fitness strategy. Why does it have to be one or the other?

i agree with this ^^ if your body gets used to one way of working out it will start to store the nec. foods to held fuel u for ur workouts... mixing it up challenges ur body!!!

Someone once told me though that fast workouts burn carbohydrates whereas slower training burns fat. Any truth in this? Sorry OP, hope I'm not butting in..

Very little truth - the so called fat burning zone is sooooo last century!
In reality your body is burning both glycogen (a form of glucose) and fat at all exercise intensities - at lower intensity your will burn a higher proportion of fat, the higher the intensity the smaller the proportion of fat is burned at the time of exercise.

The reason I highlighted proportion is that it's the overall calorie burn that makes the difference.

Really you should be training at the intensity that suits your fitness goals and the time available to exercise. I do both on different days.

Both, when used properly in a well-designed program.

"Do both. Variety is key when planing out your fitness strategy. Why does it have to be one or the other?"

This. Also, if you're doing HIIT for your hard 20 minutes it is far superior to 40 minutes of moderate rate cardio. A lot of people fail to realize the aspect of EPOC. With HIIT and going hard for 20 minutes your metabolism will stay elevated for several hours after you're done so you're constantly burning calories at an elevated rate

I try to do both mixing up the workouts randomly depending on time available and how I feel, or just do 30 minutes inserting 5 x 2 minute high intensity sections or sprints.

They yield different results. Ultimately though, fat loss will come through a caloric deficit. Assuming all of your diet is in check...

40min moderate workout will burn through carbs/fat immediately. EPOC gained will be minimal.
20min HIIT will not burn through any significant amount of carbs/fat but will boost your EPOC significantly allowing for a more sustained fat loss.

I would suggest 3 20min HIIT sessions a week and as many 40min sessions as you like. Just ensure you are maintaining proper caloric intake through this.

Don't "major in the minors."

Eat a little less.
Move a little more.
Succeed.