Q&A: Treadmill efforts/Adding run days/Training balance/Back spasms

Is there a higher perceived effort when running on the treadmill? If so, why?

As a low mileage runner, how can one add a running day without exceeding capacity limits? How can we do this safely?

When factoring in the 80/20 intensity rule. Can low intensity cross-training, hiking, walking be factored into the 80%?

I always think I need to follow a run training plan. No matter what distance, for the variety (sprints, hill repeats etc). Is this wise?

Should back spasms follow the same return to run principles as other running-related injuries?