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Hal Koerner's Field Guide to Ultrarunning: Training for an Ultramarathon, from 50K to 100 Miles and Beyond
amazon.com
Not every “hard” workout has to be a killer session. You can sneak in more quality without going to the well by stopping at a track, on a hill, or an obstacle-free stretch of road toward the end of a run and doing a few up-tempo repeats between 200 and 800 meters.