So once a week along with my other strength and cardio workouts, I do what is called a Zone 3 workout. That's when I run or stair climb my way up to a high enough heart rate that I am pushing myself hard, while still burning fat rather than carbs.
This week my workout was 4 repetitions of 20 min at Zone 3 with 6 min rest, plus warmup and cool down. It feels hard without being too hard, if you know what I mean. I did this workout on the treadmill, and ran 10 miles in about 2 hours of intervals.
(Ignore the HR glitches in repetition 1, I think someone walked by my treadmill wearing a HR monitor)