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I have been running for about a year now, and am at a point now where I am able to run for 15 km. But now I have a problem. At the point where I reach 15 km, my heart and lungs are fine, I'm not even out of breath, but my legs aren't strong enough to go further. Either my calves start to ache, or my knees start to pain, or sometimes my upper legs start to become weak.
Question: How can I strengthen my legs so that I can run further, and also faster?
This question is about training for > 15 km running, and more about endurance than speed, although speed is also asked. I require tips / techniques to be able to run longer / further, not so much faster. I would like to one day be able to run a full marathon.