Humans are built to jog; our ancestors are thought to have hunted this way across dozens of miles at a time. Cool spring temps also mean more oxygenated blood for your muscles.
Quads, hamstrings, glutes, and calves all work to keep you moving, but as your distances increase, running demands more of your core to keep you upright and stable.
Amp It Up
Scott Berlinger's clients run at a comfortable pace for 10 minutes and then at a faster, but sustainable, pace for two and a half minutes. Repeat this drill four times. As you get stronger, try cutting some of the slow periods to five minutes. When you're ready to truly test your mettle, begin decreasing the length of the slow periods as your workout progresses; you'll be pushing yourself hard when you could use the longer rest.