I am a coach who utilized Double Threshold work with my athletes this season. Wanted to give my 2 cents. We used a lactate monitor most workout days but it definitely gets cumbersome.
I found that Marathon pace for the first sessions were WAYYYY too fast. Lactate levels went up into the 4's or 5's mmol where my goal was get them closer to the 1.7-2.2 range. Tinman moderate pace ended up closer to their actual levels so that is what we went with since I couldn't test all my athletes after every repeat.
Afternoon sessions the goal was to get closer to 2.3-3.0 and occasionally getting as close to 3.5-4.0 then gave us a little more flexibility with workouts. For 400's, my athletes hovered between 5k-10k pace. It is crazy how quickly lactate is either accumulated or buffered by doing the 400's. A second or two too fast would bump the lactate up quickly.
Once racing started we would only touch up on double thresh work every other week.
Also. Your easy days need to be EASY! Easier than you think.
I do not coach Jakob.