It seems that, in this country at least, many years ago the club running scene fell in with the dogma that longer is better. As soon as you've completed a 5k it's time to tackle a 10k, then continue on to the holy grail of the marathon. Then you can call yourself a runner. Which is fine of course, but what of the runners whose ultimate goal is to fulfill their potential and not just cross off items on their bucket list?
Well I suggest you focus primarily on the distance at which your training effectively supports. Take your time moving up the distances and do so when your training enables you to do so whilst maintaining sensible training principles. I'm a believer that enjoyment is closely linked to accomplishment, proficiency and staying injury free. You'll accomplish more if you are training effectively for the distance you focus on, you'll only really become proficient if you've given yourself time focusing on the distance allowing you to adjust your training to see what works for you and, of course, you'll have more chance of staying injury free sticking to the principles and not being forced into in appropriate training forced upon you by your distance (a 20m long run as part of a 30m training week for example).
So, some generally sound training principles to start - I'm not including marathon training here as it's a different beast):
1) A long run that is longer than your race distance and at least 6 miles.
2) A long run that is 20 - 30% of total mileage.
3) Running at least 4 times a week.
4) Quality mileage of between 15-20% of total mileage, not more.
5) Quality work should be preceeded by at least 15:00 of warm up. Let's say 2 miles.
6) A quality work warm-down of at least 0.5m.
7) For most runners 2 quality workouts every 7 days is sufficient.
8) For distances beyond 10m you'll also want a midweek long run of about 66% of the long run mileage (and it's also a good idea for 10k downwards as well).
We can argue the toss over the details of these in certain circumstances but they are used to illustrate a general principle rather than being the point of this post. These of course are minimums - it's desirable to use a progressive approach to mileage over time even if your chosen distance doesn't change.
So, for someone to start training effectively for the 5k would require an absolute bare minimum of about 20 miles per week minimum, for example:
Long: 6m easy
Q1: 2m easy + 2m quality + 0.5m easy
Q2: 2m easy + 2m quality + 0.5m easy
Easy: 5m
For 10k, 28 miles:
Long: 8m easy
Q1: 2m easy + 2m quality + 0.5m easy
Q2: 2m easy + 3m quality + 0.5m easy
Easy: 6m
Easy: 4m
For HM, 40 miles:
Long: 14m easy
Q1: 2m easy + 3m quality + 0.5m easy
Q2: 2m easy + 5m quality + 0.5m easy
Midweek long: 9m
Easy: 4m