is 20 mins a day of cardio enought? i weight train 4-5 days a week and at the end i run for 20 mins is this reasonable?

20 mins is not bad. Any cardio is better then no cardio but if you can try boost it up to 30 mins, and your doing it at the end of your workout which is also good. 30 + mins 2-3 times a week is considered good.

Leeroy is right, obviously anything is better than nothing, but I'd agree you should bump it up. It can be simple little things like adding a 10 minute walk in the morning, afternoon, or whatever. I ride to the gym or pool and that helps too.

If you are already weight training on the same day you are doing cardio then I wouldnt worry about adding more cardio, you are already doing enough. If you are doing cardio on days you are not lifting, then I personally prefer 45 minutes to an hour.

Believe it or not, you're doing cardio all day long. So any time added to controlled cardio is good.

On weight lifting days, I usually go with 20 minutes as well. On non-lifting days, I go with around 30 minutes.

But it's really up to you. But remember, doing prolong cardio can cause muscle catabolism - the act of your body breaking down muscles. But you need not to worry about it because it only happens when you're doing like a 2 hour session or something like that.

Sorry for kinda stealing your thread but this goes along perfectly about what I was wondering.

So if I'm running 5-6km's in about 20-25 minutes twice a week. Then followed by a weight day the next day I won't see any muscle loss?