I started doing Billat 30/30 workouts twenty years ago. I think they helped me a lot as a low mileage, age group 800m runner. Years ago, I would structure them as 134m running, 66m jogging to cover 200m to start the next rep at the 200m mark on the track.
The key, I think, is that this workout let me accumulate more time o/a a 6min/mile pace than longer intervals.
After the USATF Masters in July, I'll return to Billats and do something like 35/35 or maybe 30/40 in two sets of 12 reps. I'll adjust the time so I can hit each 200m at the start of the next reps. It teaches me how to be consistent.
One of the unheralded benefits of Billats is that if you pay attention instead of just hammering reps, you can experiment with ways to relax while running at a very fast pace. It's not a huge benefit, but in the 800/1500, finding a second here and there can make a difference.