In Australian running folk-law there have been some time tested and strong held beliefs around training theories and approaches. Many of Australia's best runners, from Deeks to Mona, Kerryn McCann to Nicole Carroll, together with many track stars down to the 1500m event, were disciples of what has been referred to as the "Australian model" of training, and also referred to as "complex training". The irony is that, to look at the actual details of the training you'd actually see that it was simple and repetitive. But it worked.
In these more 'modern' days, people are still doing hard running sessions, but they are an integrated part of a periodised approach, with the type of session changing as you progress towards a goal event. This makes complete sense on a number of levels, and is producing results. And for runners there is the enjoyment and stimulation of sessions that vary from week to week...but there is still no denying the benefit of the simplicity of the "Australian model".
I had the fortune of being exposed to this approach during about 1998-2000 as I trained with 3 women who would end up representing Australia at the Sydney Olympics (1500m and 5000m), under the great Chris Wardlaw who might be considered the "gate keeper" of this model. And I ran several of my best times during this period. On Tuesday we did Mona fartlek. Thursday was Deeks quarters (8 x 400m with 200m float). Saturday was a progressive 3 x tan laps. Sunday was a long run. Repeat ad nauseam.
As you got fitter, you didn't lengthen the sessions...you just ran them faster. The difference between "off" season and "on" season was the intensity and focus you bought to the session. Everything else was much the same. The complexity is in the simplicity, and vice-versa.
While this model is not for everyone - I can see pros and cons in it - the main take away is that training sessions don't need to be complex to be effective. You really only need a small library of training sessions to be a great runner. In my coaching, the sessions I plan are far simpler...and easier to remember and execute...than they used to be. There are times when some nuances and specific details are important, but mostly they are not. Hence why I now describe session effort as either Yellow, Orange or Red. Simple.
So, don't over think your training, and seek out the 'magic' training session...it doesn't exist. Keep it simple and you will (still) improve.