You must be in the top 7 runners on your team to be on Varsity.
Why was cross country invented?
Technically, it wasn't invented. Running has been around ever since people. There really is no better reason to say that cross country was invented as distance running competition.
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.