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it depends what you are aiming for do you want aerobic or anaerobic increae, im guessing aerobic which means: the minimun time for workout effect is 20 minutes but 20 minutes must be in your training zone for aerobic which is 220 minus your age from that answer, you calculate 65-85 per cent of it and that will be where you want your heart rate the whole time(if you are already fit you should aim to stay around the 80-85 mark. If you have any more questions or need to know more feel free to ask me

Thats not exactly a fast pace, I run 5k in under 20 minutes.

If you want to burn fat, you have to get the glucose out of your system so that you can burn fat. Do a hard workout for 10-12 minutes, then do 20 at the pace you are doing now. You are using up your glucose anyways, so get it out of the way first and then get to the fat burning.

Eat something afterwards so you have something to use as energy, because you will be completely out of energy until you do. Dont worry, youll use it up and it wont become fat.

Real Spartans run. They don't use bikes, or ellipticals. They run.

Of course, if you can't run for reasons like, you're 'heavy' and it's too hard on your knees, than by all means use a bike. 20 mins on a bike at a good pace will work wonders.

Not only will you burn calories, you will build muscle also.