If you use the 20 x 400m in a smart way there will be no stagnation. When I coached you you improved fast on the treadmill but didn't follow my instructions outdoors when to follow the level 5 k pace that's necessary to get the best result. Instead you ran way too fast at the first reps and destroyed the positive outcome of the workout that always occure if you do it proper.20- 25 x 400m is a very common workout among the Kenyan runners and when they can't use their tracks due to heavy rains they take it to the roads and run a structured fartlek instead of 20- 25 x 1 min fast with 1 min jog between.The main reason with the 20 x 400m isn't to raise your maxO2. Instead it improves your energy efficiency and biomechanical ability at just that 5 k race pace when you repeat it week after week. To raise your maxO2 up to highest possible individual level is instead a total concept that also needs lactate threshold and easy steady runs in a very good mixed way to reach individual perfection.