Exercise is a very general term - depends what one does and where one's body is at. Water exercise (at your own pace) is great for fibro.
I exercise regularly because in the long term it helps. The more I sit, the more that's all I can do. Sometimes "the first step is the hardest" but I need to move my joints and muscle. I'm going to be in pain most of the time anyway, and some days I can't do much, maybe just some gently yoga, but it helps. I do step aerobics, lift lighter weights than I used to, bike, walk - I like variety. Sometimes the first 10-15 minutes are hard, I need to warm up a little longer and stretching & rest after is important. Pacing right ?
The more regularly one does it, the less difficult it becomes but with fibro one has to start slow and gradually build - and I think hydration is key too, as well a a decent diet.