Hey @Chris_the_viking,
As @gally24 said, welcome to the forum! 
This is an interesting question, and it seems like the conversation so far has been mostly productive. 
Cycling and running have a lot in common, but you’re right: cycling is much less hard on your body, which makes it a great cross-training option or a great alternative to running for those who desire lots of training time and are prone to running injuries. 
I’ve gone the other way than you, coming from a cycling background and picking up running over the past few years. It has changed how I think about things quite a bit, so it’s good that you’re coming into this sport with some different ideas. 
With running, the limiting factor is typically musculoskeletal injuries that start to pop up with excessive intensity in training. Cycling, on the other hand, is less injury-ridden (at least directly from training stimulus 
), but the more common training setback is from overtraining/underfueling.
The symptoms of overtraining aren’t immediately noticeable and typically compound over time and are only very obvious by the time you’ve already done some damage (not too dissimilar from a running injury in that way). While you may feel really good when jumping into training on the bike coming from a running background, it’s important to know that, like many on here have already said, more =/= better. There is a proper stimulus balance for each of us at any given moment. It’s always best to start small and incrementally work your way up in volume. 
In terms of “quality” sessions, I’d argue that each session of the week is of quality and has a purpose. The key is to complete each workout with quality and consistency to get the most out of your training. This is what your coach was talking about when encouraging you to run easy on the easy days to get the adaptations specific to that type of training while also having the energy to knock your hard workout out with good quality the next day to get the most out of that session.
We typically recommend no more than 2-3 hard workouts a week for each athlete. One longer endurance ride (much like a long run) a week can be great if you can manage the stress long-term, and the rest of your week should be very easy riding. There’s no need to do “doubles” like you might when running, as there usually isn’t an issue getting enough training stress in from a single session on the bike.
It doesn’t take much to become saturated with high-intensity stress, and often, at a certain point, doing more easy riding is going to be more beneficial than doing more hard riding. Those double-threshold athletes are likely adaptation/recovery freaks and can handle a ton of high-intensity stress and recover/ adapt productively. 
Again, each day has a purpose, so it’s important to stick to the plan (including recovery weeks) if you want to see gains compound over time long term. I’d recommend using TR to get on a custom training plan and seeing how you do over time. You’re already off to a great start! 
