and how long should i do it or what distance should it be?

running: go at 4.0 mph or higher...and don't stop until you've burned 500 calories or more.

Use the "calories burned" calculator here to see what the difference in various activities is http://www.freedieting.com/tools/ideal_b...
Both of those are good. The elliptical isn't as hard on the joints as running, but running probably burns more calories.
Do both so you don't get bored doing just one or the other.

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elliptical

running and walking for 1 hour a day helps you lose fat even if youre not on a diet

Running gets my vote. Aerobic exercise gets rid of fat, and for that both exercise types are good.
As for how long or how far, that depends on how fast you are going.
Aim to be at a speed where you could speak to someone next to you. Any faster then you will probably be working anaerobically, which won't aid weight loss.
If you can't get on with running, go for the elliptical which is also a great cardio workout, but with less impact on your joints (as suggested already). For the speed, the same advice applies.
A heart rate monitor would help you find the best effort level. As a rough guide, you'd want your exercising HR to be 220 less your age, multiplied by 65%.