I got fit doing mostly exclusive steep up hill walks while hurt and followed with 15 40 5 k track as a 38 year old.
Also had a period where I ran 5 min pace on my tred and made my workouts just extensions of the distance I had made. Went from 3 miles to 10 k in about a month. So it was 3 4 5 6 10K.Going through 3 miles on that 10 k was cake, when I did it last I was balls out. That was followed by some good races but ended up really Oveertrained later that season.
Lots of experimental training, some good, some not as much.