In the late 1990s Bill Dellinger served as the interim head distance coach for 6months at Wake Forest while they were recruiting a new coach. I got to see his “Oregon System” which I presume had been in place for decades. It consisted of 2 workouts a day every Tuesday and Thursday and a race or single hard session on Saturday. The workouts were shorter but more intense than the double threshold stuff but still the same principal. Here’s the breakdown:
Tuesday am - Oregon drill - 3 x 1000m done easy (3:30), medium (3:00) and hard (2:40) - these times are rough estimates for what the avg runner was hitting (mostly 14:20 ish runners). Rest between intervals consisted of 10x 100m strides done easy, medium hard with 10s between strides.
Tuesday pm - 4000m continuous running alternating 400m at 5 k race pace with 200m at tempo (a variant of the old pre workout)
Thursday am: Oregon drill
Thursday pm: 2400m continuous running alternating 300m at just below mile pace with 100m at just below tempo pace (ended up running ~5k pace for the 2400m)
Saturday: race or more traditional intervals.
The wake guys improved massively with this training. They ended up with three guys running under or close to 4min and finished 7th in XC the next year.
I’m pretty sure this training was what Dellinger used in the glory days of Salazar, Prefontain etc at Oregon. Maybe this was the dawn of the two workouts a day philosophy.