These are the staple diet
of long distance runners, although (particularly
in the early part of a training cycle) they are
useful for all runners and most other sportspeople.
The length and pace of a run
will be highly variable dependent on a range of
factors
Athlete's Fitness
Is this to be a steady or hard run, or even a recovery run?
What is the athlete’s main event? There is no point in a sprinter running 15 miles, for example.
Time of year - again a sprinter or power athlete will find hard runs useful in the winter, but may well only use steady runs as a recovery in the summer
What is the level of the other training being achieved that day / week?