how much time between each? I’d say probably 5k pace, with clarifications.
I once read, and now like to tell the people I coach to “run as fast as you wish, recover for as long as you want, and change it up” keeping in mind fartleks are technically non-structured. But, when I’m coaching the workout, I usually have a set “on” and “off” time (which I guess makes them intervals, technically). And based on that it depends on the pace, often mile pace, often 5k pace.
My “on” bursts vary from 15 seconds to two or three minutes. Between bursts, “off” time often equals 2/3 of the on time. The shorter the burst, the faster the pace.