fast walk or jog

ive started my first day of cardio,i did about three miles jogging at a low speed,i see people do cardio for 45min+ which is what im aiming for but my three miles only lasted 20 mins and i couldnt do anymore but i know im not fit,my legs were sore and lungs very sore does this mean it is too intense for fat burning.
i need to get a hr monitor which will give me a better idea but is it better to power walk?and should you be out of breath and tired after it cardio for fat burning,thanks

If you are only doing 20 minutes thats fine until you get condition & lung capacity increases. If you did that on empty stomach and had high intensity 20 min is okay , remember if it is done 4-5 days a week it adds up. Power walking is good as a warmup but not too good for burning the calories unless it is a long time. Just my 2cents

ok bro thanks for the info

Definitely do cardio on an empty stomach..much better results.