I had some pit in my training, couldn't run fast workouts, even relatively fast pace (not even marathon pace) for 13 miles felt rather bad and after the run I had somewhat feverish feelings. We made a week off with my coach, then I resumed training. Now I can do the fast workouts more or less normally, but my HR during easy runs are systematically increased. Usually my easy HR about 150, now 160. My HR for a hard work is 180 or more, up to 200-205. I'm 30 years old and have 35 min for 10k, my heart and stroke volume was "athletic" by general medicine standards already 2-3 years ago. Running feelings are more or less normal now (it is base period so not too good but not critical), as if HR is normal. I have unstable daily / sleeping routine (but not lack of sleep), maybe some sort of frustration. Coach told me that I can't do hard workouts now with this HR problem, but is this a real problem? He told maybe I need to run by HR under 150, but does that make sense consider that pace will be pedestrian like 8:00/mile and worse? Now the pace of my easy run is about 7:30/mile. For example, today I ran 7 miles 7:13 pace 161 HR. Is HR a scientifically sound and meaningful indicator at all? Should I keep running by feel?