Now I'm talking mainly road / flat here, so it's mainly for those who don't live near hills or do a lot of their training on the flat.
In order to keep my CV-fitness up, I do a variety of run-lengths and speed sessions and I got to thinking the one that hurts most - and is therefore probably best for me - is around 800m-1.5km. (like, I'll do 4x1km with 500m 'rests' in-between).
I'd hate to be a middle-distance runner, it's just a killer: anything under that distance you have the end in sight pretty quick, anything over that you run at a pace not too far off your long-distance pace.
Pushing it, eyes-out for a kilometre is just hell in my opinion - and must be v.effective.
Merry Xmas by the way...