Bob Glovers charts are pretty good for this sort of thing (author of "The Runners Handbook" and "The competitive runners handbook"). He bases them on percentages of world bests.
What he calls a "basic competitor" is a mid-pack runner prepared to train diligently and intelligently. I list his 20-34 & 50yr old age range times for men.
5k - 25:56, 29:03
10k - 53:56, 1:00:36
He does go on to say that with proper training you could go on to his "Competitor" category. I think he means getting a bit of coaching by proper training and being in the top 3rd of the pack. (60% of world bests)
5k - 28:47, 32:57
10 - 44:57, 50:24
To be honest I think he is perhaps being a bit conservative but I'm not in the best position to tell being quite well suited to running (scrawny and 5'9"). I reckon my lack of hair on top helps with the streamlining as well ;-)