Recovering from illness

I have been running consistently since March and I saw some noticeable improvements.

Then I got sick 3 weeks ago. It wasn’t even that bad of an illness.

Have started back into running a good week or two ago and damn it feels harder than before. Have been doing some easy runs mostly to not overdo it right away. Today I did some short fartlek in the end of the run. My heart rate spiked to levels I didn’t even see during my vo2 max sessions before the illness.

Is that normal? When will it come back to me?