When im running i have a variety of variations of a few basic courses, usually run between 8 and 16km. 8 km takes me 38-40 minutes but it has a couple of very big/slowish hills which knock the time up again. a bit over ten km i usually knock off in 47-50 minutes, depending on weather etc. It's only when i push up above that 16km distance that the times start to increse in how long i run per kilometre, but then i do much of my speed work at soccer training twice a week, so usually when running im not pushing myself too hard, except up the hills, just doing it to run decent endurence into my legs.
The racing tiem compared to training time is a huge difference, when i raced 5km last year i did it in 17.55, even when i ran a half marathon on pretty minimal training i did that in 100 minutes and 5 seconds (that damn close to under 100), but you definately need those kilometres to be able to confidently run the distances well and thus know you can kick on at a certain point.