That's your base line for now, but you'll get better. However if 4 miles x2 a week is hard, cut back, that's your new baseline.
It's hard to be philosophical at times - and funny you mention bipolar as my friend with bi polar and I have compared conditions and there's similarities in that I can go up and down; I get carried away with how much energy I have when well and it affects my mh when I'm running out of energy.
I recognise the need to manage your bipolar with exercise - apart from it been really good all round I think it's also feeling in control of something, which with thyroid stuff you're often not till in the know about the condition, what's happening and on top of medication. But then I guess it's similar with bipolar too!
However, it'll be doing stuff to your body which will take time to recover from physically. When better id get gp to check Vit d, ferritin and b12 as I believe all can be affected by it and optimal levels will help your muscles, nerves, and so mental health recover better too.
Yes scan will help understand more. Patience is a virtue, take each day at a time, and as Douglas Adams says Don't Panic :D (says me.....)
I think I'm able to recognise that this shit happens for a reason and I can learn new shit from it. Which I have, not least that I've got hypermobility syndrome but also I've bizarrely learnt how to do proper front crawl just by watching a guy swimming whilst I was sitting in the sauna (not swimming) 