An athlete reached out stating that they feel as though they have been holding station a bit and were wondering where the gains had gone.
Hopefully my response is of interest or use to a wider audience.

Improvement comes from application of overload stress (a) and adaptation.
Repeated bouts (b).
Over time (c).

More stress, more bouts and more time will show results but the absence of any of those things will lead to nothing.

No progress will occur if you look for a quick fix (no time) (c).
No progress will occur if you don’t train often enough (no repeated bouts) (b).
No progress will occur if you don’t apply sufficient effort (no stress) (a).

Generally the most common cause for stagnation is that athletes neglect the ‘harder’ work (no stress) (a). We do this recreational thing because it’s fun and hurting isn’t a lot of fun so we do it less often. Before we know it we’ve run several weeks and can’t recall the last time we were properly knackered after a good session.

Are you “running” or “training” (a)
Sometimes do some training. Most of the time just running is fine.

Easy stuff easy / Hard stuff hard (a).

Make around 80-90% of your running easy but by crikey you gotta do at least 10% hard (a).

I’m not assuming you are lazy but giving you the long range view of what we see most often.

A little of what you are alluding to can be related to ‘running age’ (years spent running) (c). The ability to maintain a ‘moderate’ intensity for a LONG time is more visible in ‘experienced’ runners. We can get you a lot faster in a shorter period of time and your 5km time can improve significantly however your ability to turn that into a comparable marathon time may be limited to the depth of your cardiovascular fitness which we can only develop over time (c).

Lastly, consider your training frequency (b). If you’re only out 3 times a week you’ll struggle – even following a good programme – to lay down sufficient bouts of stress for adaptation (b) to show progress. Pro’s train twice daily most days (b). That’s the far end of the spectrum but the more you want from your training, the closer you’d want to be to that end.

To be clear, I’m not saying that you’re lazy or you should train twice daily. Just know where the true gold standard is and ask yourself how close you are to it.

Hope this helps.