I ran 5.5miles on Feb 13th and my HR did not go over 169. 6mph on the treadmill. Today I did 3.25miles and my HR got to 172, I did not get to finish my run. Same pace. I told myself once I hit Z5 I would stop. I shouldn’t be in worse shape, Ive only done more running not less. So my question is, am I in worse shape or is there something I don’t know about HR? That’s a pretty drastic difference. I still had 2.25miles to go by the time I was hitting z5, last time I didn’t hit zone5 until 5.5miles. Uhg. Please help.