It seems the aim of C25K is more about running for 30 mins than doing 5k but I guess C230M would have been less catchy. Personally if I am going to run then I want to run at a good time (for my age) rather than for a set length of time. From the number of posts on this forum bemoaning their apparent lack of speed I am not alone. Lots of people seem to want to know how to increase their speed in order to run 5k in 30 minutes.
Aided by my somewhat nerdy interest in numbers (this is new, I blame the C25K programme) I can now share with you the following blinding glimpse of the obvious. Your speed (or pace) is simply the product of 2 factors - your stride length and your cadence. Cadence is the number of times you swing each leg forward in a minute. To increase your pace you can either increase your stride while maintaining your cadence or increase your cadence while maintaining your stride. If you increase both you will see a big increase in pace.
If the target is 5k in 30m your pace needs to be 6 mins per km or less. There are many combinations of stride & cadence that will achieve this. You just need to find the one most comfortable for you given your physique (leg-length, mobility etc). Taller runners with longer legs tend to have a longer stride and lower cadence. I am only 5'8" so my shorter legs need to move back & forth more quickly to keep up with someone who is taller.
Many running apps for smartphones will tell you what your stride length and cadence figures are (I particularly like iSmoothRun for iPhone). Alternatively you could just count your strides over a timed minute to find your cadence and measure your stride by counting strides over a known distance and doing the maths.
You can then experiment with adjusting your stride length & cadence to meet your goal. It is generally believed that a faster cadence is preferable, even if that means your stride length is slightly shortened. Be careful not to 'over-stride' as this causes additional stresses to the legs. Personally I find it easier to concentrate on keeping my cadence up and let my stride length sort itself out.
It is often said that a cadence of around 90 is 'optimal' (this is for one leg - it is sometimes expressed as 180 for both legs) however if my experience is anything to go by this may feel really quite fast for new / inexperience / unfit runners.
The following combinations of stride length & cadence will all produce a sub-30 minute 5k time if maintained for the full 30 mins;
Stride / Cadence
1.4m / 60
1.3m / 65
1.2m / 70
1.15m / 75
1.05m / 80
1.0m - 85
0.95m / 90
0.9m / 95
Hope this helps...