The workout isn’t used to inform fitness, which is the most common misattribution error in training. Workouts are performed at the level of one’s proven ability in the target race distance with an appropriate level of competition corresponding to one’s current fitness that is typically ill defined: capacity and resistance to fatigue.
The 3 workouts indicate a lacking aerobic fractional utilization with a proximate cause of global volume and likely overall average pace:
5:20 pace, 5:11 pace, 4:48 pace without maximal volume of 8 miles, 8k, and 6400 meters or 4 miles: negligible difference between miles and 1600 meters. A steep drop off occurs with an increase in performed distance and a significant inability to perform extension.
2 suggestions based upon training background notably length of time at current mileage: only 4-6 weeks are required to optimize a focused variable utilizing the law of Contiguity. Increase mileage to 85-90 oscillating within that range and add steady paced running at MP (5:35 with drop off) to MP + 15 by inserting as a warm up.
The drop off on distance increases should be 8 seconds per mile and one workout every 2-4 weeks should be a Marathon workout consisting of a mile at MP+15 and a mile at 15k to start gradually progressing to MP and 15k as your aerobic efficiency and economy both improve through repetition and once weekly barefoot strides for foundational structural strength and sensorimotor integration.