What did/do you do for speed work? I've been doing a c210k programme for 5 weeks and a few tempo and progression runs has allowed me to increase my speed by 0.4 kph, albeit from 8 kph, but an increase nonetheless.
How many sessions per week were dedicated to speed work?
No more than 10-20% of your total weekly volume. And for a begginer even less is plenty. Lots of nice easy runs builds speed