Thundercat 45 to 60 mpw should be enough to make good progress - so the hard days must be interfering, as I said, with your aerobic improvement.
Having said that, if you were running faster than 5k+3 at sub75% max then the current pace must be the result of the illness. Plus your real max might be a few beats higher than 191 (I only max out on 400 repeats).
I'd stick to 8:20 to 8:30mm for the bulk of the week, and wait for the HR to fall through 75% or lower before speeding up.
By way of illustration, I've been doing HR training for a couple of years, and when fit, sub75%max for me is about 5k+2' (for long runs) and 5k+90" (for say 5 miles).