Working out to increase fitness is going to have to involve some high intensity workouts that challenge aerobic capacity or strength but equally important for growth is recovery and rest in between sessions.
Also important to consider is other stressors, such as work, quality of sleep, anything that could affect energy levels, these things usually can’t be avoided but can at least be taken into account when planning a particular workout.
For example, over the last few days I’ve been feeling run down with a cold, had a couple of nights less good sleep, and in the last week added an extra weight training session sooo, today instead of doing a mile repeats workout I repeated last week’s 400m intervals workout. This meant shorter blocks of high effort, plus the confidence of having done it before which helped to actually get me out the door and still do something challenging instead of swapping for an easy run or skipping altogether.
As a bonus I was able to compare two similar workouts, this run felt much stronger and smoother than last week which is a real confidence boost and has fired up some enthusiasm for the more difficult session next week!