There are some nasty viruses going around at the moment, I caught one before Christmas not much more than a sore throat and a runny nose, otherwise felt quite well, until I went out for a gentle walk and my HR went up to high Z4 even though I was at a pace where I would normally expect low Z1! despite my resting HR only bring slightly elevated 5bpm or so. I took another week of complete rest, didn’t walk further than the end of my street, followed by a week of walking up to half hour, to keep an eye on my HR response. Week 3 was walks of up to an hour at a fast pace and only after that did I return to Sufferlandria.
Useful topic here on returning to training after illness Back to physical activity after Covid19 infection