Running is a bit like a jigsaw puzzle for me - I have pieces of knowledge but don't know where they connect together. For example, C25k told me to "warmup using a 5 minute walk" , subsequent Internet research told me to warmup using other dynamic type exercises. I was also aware that running the first couple of K is always the hardest. I also saw ( while watching the elite frontline starters at a big half marathon race ) runners doing multiple FAST 100 metre sprints in the few minutes prior to the HM start and basically exhausting themselves before the start and could not understand why they were doing that . Now I know - it's all about eliminating the oxygen debt that we all get during the first few minutes of exercise by moving it forward to the warmup period before the exercise starts verywell.com/what-happens-i... I tried it last Saturday at parkrun and felt much better for it .