Why easy riding is so hard for my legs?

My season's gone bad. I had serious lay-off 3 weeks ago due to infection which was a relapse from earlier illness with another major lay-off. So I've been training this season with big breaks, 4 weeks on, 3 off, 4 weeks on, 3 off etc. Now I'm recovering and riding mostly at easy pace, maintaining heart rate between 60 and 75% of MHR. I find it very hard on flats for my leg muscles. To achive target HR zone I have to apply too much force. My legs are too sore after 1:30-2:00 rides at target zone. Spinning doesn't help. I have to spin too fast and I'm not a good fast spinner.
I tested my sprinting. Max. speed is still good and close to my normal max. speed but peak HR is still very low, usually a good sprint pushes my HR to 180 bpm or higher. Yesterday I hardly reached 170 with the same max speed as before. I also tried to ride faster, rising my HR over 75%, and did it with no difficulty. But easy pace is still killing my legs. Why?