Serious runners swear by hill training. But a new South Dakota State University study finds that running intervals on flat terrain may be more effective than hill repeats for improving your distance running. When athletes performed intervals on either level ground or a 10% incline twice a week for six weeks, the level-grounders were subsequently able to run longer at maximum effort before tiring. Why? Level terrain training more closely mimics the demands of mostly flat-course racing. On flat ground, runners were able to sustain maximum effort for a longer time than the hill group could, without any more rest between intervals. Note: uphill trainers in this study made gains too. If you’re seeking more speed or prepping for a challenging course, hill training may be best, 
says study author Dr Derek Ferley. Also check out The Truth About Barefoot Running, how you can Beat Stress With Running and Why Running With A Partner Works.