Running by feel and heart-rate monitors
Anyone who reads my training diary will know that I habitually record a lot of data. For aerobic runs I'll record the course, time, pace, average/maximum heart-rate and the weather. Why? Well, it's useful to have a history of data for backwards comparison — I can see what sort of training I was doing when I was 'in good shape'. However, this doesn't mean (as an ageing runner) that repeating training blocks that resulted in PB times is the best way to train now.
I'm a huge advocate for running (and racing) by feel. I know how easy running should feel. The same for tempo running, intervals, sprinting, hill repeats, racing and long runs. One thing I don't do (these days) is 'heart-rate train'. When following Hadd training back in 2007 I used to stay strictly within the suggested zones for various training sessions. If my heart-rate went too high during an easy run I'd stop and walk! These days though, I'll start the Garmin prior to a run or race and not look at it until I've stopped. I run by feel and ignore the numbers. The only exception would be if I'm running a new out/back course and I'm looking for a kilometre point on the way out.
The main thing I like using post-run heart-rate data for is to work out my 'RS result' (for Robert Song who I got the idea from). The RS number is actually 'heart-beats per kilometre' — it's calculated by multiplying average heart-rate for the run by average kilometre rate (you can use heart-beats per mile too). Every week I'll do a steady run over a familiar course of 8 to 10k (often the Speedygeese warm-up) and afterwards calculate my RS number. I think it's a great way to keep an eye on one's aerobic fitness (besides racing a 10k or half marathon — which one doesn't want to be doing every week!). Each runner will have their own scale of what's great, average or below average in terms of aerobic fitness. For myself at age-55, below 690 h/beats per km is great, 715 is average and over 800 means I'm not fit!