Plans, paces & PBs Forget target pace for now
There is no single correct minutes-per-mile or minutes-per-kilometre pace for Couch to 5K. Two beginners can have very different speeds at the same useful effort.
Run slowly enough that the final interval of the session still looks broadly like the first. If every repetition gets dramatically harder, the opening pace was probably too fast.
Use the talk test
A simple guide is whether you can say a short sentence while running. You do not need to chat continuously, but you should not be gasping for every breath.
If breathing becomes ragged, shorten your stride and reduce speed. Walking is also fine if you need it; the point is to complete the session under control.
Why slower works better
Running more slowly reduces the chance that each session turns into a maximal effort. That makes it easier to recover between the three weekly runs and gives you a better chance of staying consistent.
Speed comes later. At this stage the important adaptation is being able to run for longer without needing to stop.
Treadmill pace
The same rule applies on a treadmill: choose a speed that feels easy rather than selecting a number you think a runner should use.
If you can complete the interval with controlled breathing and could imagine repeating it, the setting is doing its job.