I used double threshold approach in my marathon preparation, but as a 6 weeks strength block after 12 weeks base block and before 10weeks marathon block.
Can tell you my own experience:
1) I started very conservative, am session 5x6min/60rest with lactate reading below 2.4, around 2.1-2.2, pm session 25x400 & 10x3min with lactate reading below 2.8-2.9; easy days almost always doubles: 40/40 and slowly progressed to 50/50 or 40/60 with super slow pace (lactate around 1mmol), by heart rate it is about almost always below 68% of HRmax. Every easy day between doubles I felt trashed like sh@t, but during workouts day I hit all of them about 98-99% according plan and lactate level. Most scary thing was in the specific long run days: you feel trashed, but once you started fastest part of the workout everything is going, every time when I started specific workout I could not believe that can completed, but every time complete it and feel strong, magic...
2) X element on saturday from 4 reps by 45s uphill slowly built to 10-12
3) 2 double thresholds+x element + medium long run or long run in one week is very treaky beast, some weeks I skipped medium long run because could not recover well untill next doubles
4) I completed 12 threshold, 6 x-elements and 4 long runs and maybe only 2 or 3 medium runs during this block
5) it works, about almost every double session the pm one feel better and easier, if you have time between them less than 4-5 hours (my personnal feel), magic
6) strength training 1-2 times per week
7) I could not believe that I was able ever to do per week total duration near threshold more than 1hour and consistently many weeks in a row, after that 20min tempo or 40min LT intervals per week it's looking so funny and peanuts... not enough... at least for the distances more than 10km for good performance and endurance, magic
Next 10 weeks marathon block I moved little bit away from doubles and took more classic approach to the marathon: 5key marathon workouts by MPR coach+strength training: fastest one: 6x1600@10K/2min jog once per 3 weeks (my favorite workout), 10K @HM- once every 3 weeks, 16K @MP once per 3 weeks, 16Keasy+16K@MP -once per 3 weeks (most scaried workout for me), Long run (slowest one) - every week except weeks where was specific long runs. Some weeks where I had only 1 workout 16K @MP and 1 long run, I still was doing +1double threshold: 5x6min@MP+10x3min @Threshold, comletely removed X element in this block (can give some space to the second strength training day during week instead, here I have some questions: may be it is better still to leave one workout every 3rd/4th week too?). Some sessions like 6x1600 and 10K@HM I was replaced with the real race. All 16 weeks 3 times per week strides (15s/25s length every other week alternation). No any fast staff here, except strides, the fastest pace in the block is 10K workouts..
This is experimental training plan, for sure it has some errors, mistakes and bullsh@ts...😆, but until it works on my progression, I'll use it, and my body likes 10K/HM approach more than 5K/10K approach (here I got a lot of injuries).