As a runner I'm a bit obssessed with numbers (as the OCD persons Sheldon and I are) and want to track everything. I'm not a big fan of fartlek as it's not structured enough - I want to know pace, distance, time and so on to be able to track my progress. I find comfort in the numbers, I can't cope with doing an uneven reps of intervals and if I've decided to do a certain reps at a certain pace I will do them, even if it almost kills me. A friend of mine is all about the 'feeling' and going with the 'flow', as a complete opposite to me. Sure, I like my running to feel great - but I want to know how fast and far I run while the running feels great.