not if you don't strectch before you run. If you start running on tight muscles then yeah its gonna be hell at first. Once you get to 10-15km it feels comfortable cuz your muscles have loosened up by then.

You would think so wouldn't you? However the reason is actually the basics behind human metabolism. At the beginning of the run you use carbohydrates as the major source of energy, and these are really for short term instant action use. Primarily the muscles use glycogen stored inside them (and also the liver) but reserves last a matter of seconds so glucose (and other carbs) are metabolised from the blood to provide the source of energy. The energy source is metabolised into ATP via the Krebs cycle, an aerobic pathway requiring oxygen, hence why you breath heavily. It takes around 20 mins of constant aerobic exercise for the body to switch to the use of stored fat as a source of ATP. Up to this point, as the carb reserves are dropping, you find it harder and harder to keep going. Once the pathways switch and yopu start to burn fat to make ATP, of which people usually have plenty in storage, people often finds the exercise gets much easier as the energy supply is no longer dwindling, it's constant and plentiful. By the way, 20 mins is a rough guide, people do vary depending on genetics, regularity of exercise and other factors. I'm guessing after the 4 km you are in the region of about 20 mins, which as explained makes sense.

The first part of the run is always the most difficult for me, and I've been running for over 30yrs. A lot of times you just start out too fast, even if you think you're going slow. Warm-ups help. But yeah, I've tried everything and have just come to believe until I get into "the zone" I'll have to struggle thru the beginning. Good luck.