For those of you who have completed C25K and have discovered parkrun - maybe wondering where to go from here - here is a free training plan the5krunner.com/2014/07/14/... which incorporates those elements of running training that lead to both faster 5K times and also to longer runs (10K??) .
You will notice that this plan tells you to run parkrun each week at a pace 1-2 minutes slower than your PB. It also has you running shortish intervals at a pace faster than your 5K PB pace and longer time intervals around PB pace and long (40 minutes to 90 minutes) but slow longruns.
And THEN - once maybe every couple of months , you can "Go for it" ( pacer day is a good day to do this - we can all run faster behind a pacer than we can on our own)
The philosophy behind this plan is applicable to all runners - regardless of their pace now.
I am essentially using this now - 4 days per week - but my long runs will be really long and really slow - up to 3+ hours over the next 6 months using a Galloway run/walk strategy - training for a HM in July and a 35K in October.