Marius Bakken developed a double threshold regime that he thinks got the most out of his talent. He found threshold training to be giving him improvement over time. The key was to keep the lactate levels below 3.5 since above this tended to NOT give the same improvement. He also watched the norwegian sprinters at that time doing very hard days with lots of intense volume and thougth doing the same. I have tested double workouts like 2x30min threshold volume and it kindof compares to 1x45 and not 1x60 (which would be really hard). So compressing training in one day does something to the ability of conducting the workouts and it still gives the same room for recovery until the next workout day. So if you want to up the volume of the workouts, I believe this is the model instead of extending the workout length (which will reduce the quality and effort at the end) or doing workouts more often (reduced recovery time).
Just because good runners do single workouts and do continuous tempo running does not prove that this is the optimal. It just tells you what they have used as a viable method.
Marius also used a shorter time between workouts, often not more than 4-6 hours. The reasoning was that the more he compressed the longer the important recovery AFTER got. Recovery inbetween was not important at all (other than fueling up a little ). The Ingebrigtsens do morning and evening sessions so they do longer time between.
The most important is the recovery between the workout days and between hard periods. Research also show that a a short block of hard intensity followed by an easier block is better than a distributed plan in increasing performance, even when the volume of hard and easy is the same in the compared period. So I am a believer in that digging deep with tough days and micro-blocks followed by recovery is better than evenly distributed.