Low energy often comes with depression. So it's not easy as telling a depressed person to "do more exercises".
Depression is "invisible". I like to think that being depressed is like having a broken leg. You can't just get better by yourself, you need proper medical treatment.
Replace the broken leg with a generic bad back (non surgical) and it's not a bad equivalence.
Medical advice in many cases is to do things which hurt (exercise, stretching, lose weight) otherwise the back won't get better, but doing nothing will almost certainly lead to stagnation or a worsening of the condition. You can take pills which may reduce the pain, but those are not really a long term solution on their own.
I don't wish to diminish how hard it is (in either case, intentionally causing yourself discomfort is utterly exhausting in every way). It often is both possible and beneficial so be wary of writing off a school of thought entirely.