after completion of C25K.
I really like this little book by Roy Benson PRECISION HEART RATE TRAINING howtobefit.com/Polar-Precis... Download it while you can.
This book is the only one that I can find which is consistent in their advice as to how to calculate your personal training zones - and presents 3 quite simple training plans which go to a VERY high level of intensity and volume - and which are not race length dependent.
The first plan would be ideal to go on with after somebody finishes C25K - so much so that nearly 3 years after graduating C25K , I have now started this programme.