You hear this question a number of times in the forum.
I was reminded yesterday by my daughter of my coming in after my first few C25K runs and talking about how I had found it so difficult to keep going for just 30s/1m/xx insert appropriate time! Now having graduated in November I still find it hard going. The difference now is that the time and distance has got longer and further.
I think the truth behind the answer is you make it as hard as you want to. Whether you push yourself faster or further is really up to you. What I do know for sure is that during my C25K I was for most of the time going really slow! I am not a lot faster now most of the time, but those faltering walk/runs all build up the strength to run faster and further. Personally for me the slow and steady was exactly what I needed.
Today marked a couple of milestones for me. Having started C25K the last weekend in August 2017 to get fit and lose weight and coming on this forum to see "this is not a weight loss program" I carried on regardless!! What harm could it do!!
So today I recorded a weight loss to date of 3 stone and to top it off ran for 8km - it took me just over an hour and was tough going but from being unable to keep up 30 second runs to running for 60+ minutes continuously - it felt great!! Additionally a mid week 5k for the first time under 40 minutes gave me time to celebrate what I have achieved so far.
I say so far, because it's not over. I reckon I have a long way to go. Plenty of more weight to lose. More hard runs to run. More goals to set for myself and to try and topple.
Whatever your stage in C25K whether it be starting out on week 1 or week 101 achieving a worthwhile goal is probably always going to be tough going. But the feeling and sense of achievement is so good. Keep running everyone, don't push yourself too hard but set your face to the task in hand and go for it.