Much of my working life involved being out of the house for a 12 hour day. Hence the 45 minute limit. 15 mins warm up and approx 30 mins hard work. Warm up limited to 15 mins by concentrating on progressively faster efforts to settle speed and range of movement until I feel loose. I don’t bother with drills I find they have less value than progressive runs. And the motor patterns are less relevant imho.
The hard efforts fit into a 30 min period with 5-7 min recoveries depending on number of reps.
I agree about not mixing paces within a session. I feel my issues are related to varying types of training in a weekly period. Speed, speed end, tempo, weights is 4 different things in 7 days.