When you’re feeling flat, a little fast running is often the best cure. A slow 5 miles might leave you feeling more lethargic. Instead, throw in some random short pick-ups, or do a set of striders on your street once you’ve done your normal loop. Because they get your central nervous system working at a higher level but aren’t long enough to wear you down, little bursts of fast running can help you surpass sluggishness.