There's no magic workout or perfect plan to improving as a runner and as an athlete. There’s consistency, adaptability and a process. What matters more than any training plan is being in tune with you physical and mental trajectory over time and letting the art of training become more center stage than the science of it. Running is simple but it’s not easy.
Rather than focusing on exact interval session prescriptions and debating between I.e. 8x800m or 10x1k with either 1 minute or 90 seconds rest it may be more effective to ‘zoom out’ and do what you are excited for, what your body is craving or what a training group is doing as long as it is a similar stimulus. Moreover, the exact benefits of these kind of sessions are extremely difficult to measure.
A more exploratory approach such as fartlek on the trails may be more appropriate for a number of runners. Traditional fartlek or ‘speed play’ is not as common an approach as interval training prescriptions. The popularisation of interval training and the normalisation of researching different interval training protocols largely stems from the approach of the first person to break the 4 minute mile: Sir Roger Bannister. Due to time constraints, him and his coach adopted a very systematic approach to break the 4 minute mile. As a result, Interval Training sessions such as 10x400m have been engrained in running training systems worldwide. And rightly so: they work! However, for a lot athletes, they may be better off ‘playing’ with their training to stick with the sport for longer, to enjoy it more, and ultimately achieve their potential.
Over a decade before Bannister, two notable athletes who used fartlek were Gundar Hagg and Arne Anderson who were extremely close to breaking 4 in the pre WWII era. If Hagg or Anderson had broke the 4 minute barrier rather than Bannister, then ‘Tuesday Fartlek‘ might be what ‘Track Tuesday‘ is today; engrained in the running psyche. If it wasn’t for WW2, they might well have! This can be extrapolated to periodisation which stems from industrialisation and the routine of workers.
Quite clearly, there are no concrete answers to being a successful runner. It is about experimenting, learning, getting advice, trial and error, having fun and trusting in principles rather than formulas. A lot of training theory stems from social structures and social norms. It may be necessary to break the norm and follow your own path to achieve success in this sport.