20+years of fibro here OP.
Your legs/feet literally carry your body's weight around more than any other part of you does, so will actually complain quite loudly.
You say you're on your feet all day and I can truly say that that is where the tightness of the muscles is coming from. Usually when walking you transfer weight evenly from one foot to the other, so each leg rests/works/rests/works etc.
As a teacher I daresay you do a lot of standing, with minimal walking around. The longer you stand, the more weight your legs and feet carry with no relief. The tightness then begins to build up in the muscle tendons that attach to the bone which is probably a reason the pain is beginning to manifest itself in your shins. I began to find a similar problem once I became post menopause at around 55 years old.
This are the steps I take to enable relative pain free walking:
Massage problematic areas every evening - I use a cheap moisturiser with something like almond oil mixed in, and after a bath spend 10 minutes or so massaging.
*Gentle stretching routine 10 mins - morning and night.
*Ditch the heels - my shoe heels now are no more than 1.5 inch in height, and it definitely takes the stress off the shins.
*As pps have said, I also use a weighted blanket on my legs in bed, only mine isn't a modern thing, but a feather eiderdown which I've had for years.
*Wheat bags - heat helps muscles to relax, and as my feet get very cold even in summer, these work very well for me, as cold feet equals no sleep..... for me anyway.
Vitamin D -discovered I was deficient. So take a regular dose each morning - has to be morning as it helps to regulate sleep too.
Vitamin C - This has been the biggest improvement for me. I can now after 4 months of daily doses stand for longer without pain, ride a motorbike for an hour or so before needing to stop, still enjoy gardening. walk almost a mile before needing to sit down. etc
*Keep the weight down
*move as much as possible - don't stop bending, stretching, walking, dancing, just do it in smaller amounts as and when.
I regard Fibro as a shapeshifter, never the same more than 10 minutes in a row, so almost impossible to get to grips with - or so I thought in my early painful, scary days of living with and accepting this. But in all of it I simply refused to roll over and cave in because I had - and still have - a life to live.
Get annoyed with it, refuse to do what it dictates to you - at least most of the time - listen to your body, try everything you can because what works for one doesn't necessarily work for all. ....and don't apologise to anyone for having this. If they don't like it, don't believe it, or whatever then tough, it's their problem not yours.
You can and will get through this - I honestly wish MN had been around when fibro hit me - there is so much support here, so keep asking .