Here is an explanation from Runner's World.
"Fartlek Workouts are not only fun to say out loud, but they're fun to run. Fartlek is Swedish for "speed play," and that is exactly what it’s all about. Unlike tempo and interval work, fartlek is unstructured and alternates moderate-to-hard efforts with easy throughout. After a warmup, you play with speed by running at faster efforts for short periods of time (to that tree, to the sign) followed by easy-effort running to recover. It’s fun in a group setting as you can alternate the leader and mix up the pace and time. And in doing so, you reap the mental benefits of being pushed by your buddies through an unpredictable workout. The goal is to keep it free-flowing so you’re untethered to the watch or a plan, and to run at harder efforts but not a specific pace."
10 minutes--AMT warm-up
35 minutes--lifting--leg extension, squat rack, sumo squat, reverse squat.
45 minutes--treadmill, easy run--5 miles