I've spotted on my long runs that if I run to a steady heart rate, my pace (not unreasonably) drops off - especially after a dozen or so miles. I'm interested to know what the physiological reason is - is it to do with scraping the glycogen barrel, or deterioration of my running style (& efficiency), or dehydration, or what? And in a marathon, is it better to maintain a steady pace (as I'm given to believe) or constant HR (does this equate to constant effort?)? I don't intend to race with an HRM, by the way, just train.
Thanks, you lovely lot.