Thanks for your update. You did so well today. A careful start to getting back to work and wonderful your employers are listening to you.
I am obviously the world's worse at not taking my own advice, but you really have had a good day, work, the gym, a calming restorative walk. You are aware of the good things you can do to help yourself. Anxiety doesn't always seem to follow a pattern, no matter how careful we try to be with ourselves, it's sometimes quite irrational, a sneaky bugger if you like.
Why not write yourself a bit of a diary? Nothing too deep or dramatic but just a note as to how your days go... Yes, the anxiety hit you for no good reason, but also note your positives, of how you were able to cope with your day. You were working strongly to help yourself. It may help you see the good - it's very natural to see the bad bits, and not remembering your own positive strengths.
X