I've heard that you don't gain anything by running faster than 90% of lactate threshold other than it hurting. I started incorporating heart rate into my running. Today I did an easy 6 in the morning up and down hills and I tried to keep it in between 140-150. It hit about 150-155 up the hills and sank to 140 or just below on the downhills. Think I averaged 8:50-9:10 which is very slow for me and it was tough not to speed up. Going to do another 4 this evening. Is this heart rate too high for that pace? I have always been told 150 is really working. For reference, I run a 17:45 5k and am looking to break 17. I've always been an aerobic runner but this just seems strange.