In reply to Darren09:
Depending where you're starting from a commonly applied rule is
Increase overall distance by no more than 5-10% each week
Increase "long run" distanec by no more than 5-10% each week
Alternate hard/ easy work outs
Either alternate hard easy weeks, or Increase distance for 3 weeks, then in the fourth week have a "consolidation" or easy week where you step back a bit in intensity and distance - a bit like this:
50
55 (up)
60 (up)
57 (consolidate)
63 (up)
69 (up)
76 (up)
70 (consolidate)
Don't do a big increase in speed sessions (high intensity work) at the same time as increasing overall milage.
The massive motivation you get seeing rapid improvements when you up your training - can really entice you to ramp up quickly - but (depending on your running base) it can be easy to pick up niggles/injuries (knee problems, etc)