I used to hate Fartleks, back when my running was ruled by my watch and HRM. Distance and time and pace told me how well the run went, which to a point is fine and good and certainly has it's place. But being capable of gauging your effort by feel and let go of the data is useful and I think one aspect of being a complete runner. Not necessarily a good runner, but more well rounded, which usually tends to lead to being better. And for some irony, below is what a graph of my fartlek looked like today. This is elevation and pace. Note: I'm not fit and "hard" is relative. I was blowing up after brief pushes to just 9:00 pace on some climbs. But I was pushing, which was the point. I'm not bummed or disappointed because I'm not fit and my fitness is a result of how hard I've worked on my running. If anything it motivates me. To quote Tool's Schism- "I know the pieces fit 'cause I watched them fall away" which I love because it fits for me here. I've been fit. I know I can be fit again. And I know how to get there. But like anything, on paper it's simple and knowing how is more difficult than the doing.