Fartlek is a Swedish term for “speed play.” This informal version of interval training is simple, effective and dare we say … enjoyable? And not just because "fartlek" is fun to say.Exercise columnist Gretchen Reynolds is here to vouch for this often-overlooked, decades-old practice. Reynolds, who helped popularize the 7-minute workout, has found that a growing body of research points to the benefits of this approach, even in small doses. According to exercise scientists, it can improve strength, endurance and longevity, and may even lower the risk of dementia. Read more: Don’t like to exercise? Try this simple, science-backed trick.Being in shape is better for longevity than being thin, new study showsToday’s show was produced by Elana Gordon and mixed by Sam Bair. It was edited by Maggie Penman. Thanks to Anjuman Ali.Subscribe to The Washington Post here.