Today my postural tachycardia seems much improved. It is normal both standing and pottering around the house and when I went for a walk it was within the normal bounds of light exercise (some rise is expected). What was most significant is that it dropped down to normal immediately on rest, which would seem to me to indicate a normal response to exertion. It even has slightly improved when walking upstairs, though it still doesn't like me raising my arms (getting dressed always causes tachycardia). I've also been less fatigued the last few days, despite walking further, and I managed nearly 2 miles this morning.
Obviously I know this might not last; I got covid nearly 10 weeks ago and know how symptoms vary from day to day. But these are all hopeful signs that my routine of increased fluids and increasing light exercise are helping.