IS running 5km in 20 min good?
The answer to the question is as wide ranging as people are wide ranging. In general, for a person in "pretty good" shape, i.e. runs a couple times a week, if you can maintain a 5/KM run rate you are doing pretty well.
That would be 25 minutes for a 5k. 20 minutes would be considered good shape and when you get into the 15 minute range you are doing very well.
I have a Garmin footpod and a Garmin GPS watch. For me, I calculated the answer to be 5,050. I am 47 and weigh 170 pounds - this was running at 9:41 mile/minute pace. The answer truly depends on one's stride length, though, which for me is 6.493 feet/stride. Hope that helps.
Average time for 5-k a fit person?
I t depends how fit you are. I consider myself to be somewhat fit and i did a 5K race last year in 24:43. The other day I ran 3 miles in 22:29 which is alot better from last year. So to answer your question i would say anything under 26 min would be decent.