@conduitoffortune it makes sense to me to use intervals to ramp up to a new maximum running time. One of the comments on the c25k app was that when your body is ready often your mind is still afraid of the idea of the next step.
However what doesn't make a massive amount of sense is when the running time goes down instead of up. I don't see why the time shouldn't go up a little bit every time.
So after having done 20 minutes in a single stretch, the next run can be 21 minutes but divided into 3 intervals and that doesn't seem too scary as it is barely any more than you previously did. Then the next could be 23 minutes divided into 2 intervals, and then for run 3 the 25 minute run seems totally doable.
Certainly when I start trying to ramp up from 5k to 10k I will use intervals - I will start doing hour-long excursions which are initially 50% walking and 50% running so I will be running 5k (which I will have already mastered via c25k) but gradually build over many weeks to eventually being 100% running.
What doesn't make a great deal of sense to me is that there doesn't seem to be any of this structure in weeks 7&8 of c25k - they just repeat 25min and 28 min runs 3 times.
But there's nothing to stop me from running an extra run with my own timings in between "official" runs, or inserting an extra minute or two of running here and there, if the c25k timings don't work for me. It's not like I am going to get into trouble for not following the instructions exactly.