My question is pretty straightforward but I understand the answers will be widely varied. With that said:

How did the Pros train before they had power meters? More specifically, what did training plans look like when coaches were training on HR data only?

I started using HR near the end of last year, and I recently acquired a turbo trainer. did I've been lifting weights since November, lost a bunch of weight, and gained tons of explosiveness and power. The problem is, I can't quantify how much!

If I had a power meter, this would be a simple task, just look at my peak threshold in each zone, 5s 20s 1m 5m 20m hour, and wouldn't that be dandy. I only have a HR however, and I'm doing a ton of HIIT intervals and still lifting,

What is the best way to QUANTIFY my improvement with the data I have available now? How did the pros create a training plan without power data?

Variables I can measure that matter:
-cadence
-perceived wheelspeed on the trainer
-trainer mileage
-trainer time
-trainer resistance
-gear
-performance in plyo/sprint workouts
-most important: heart rate