I started doing intervals two times a week. It's completely horrid. I just snagged two intervals to do from Joe Friel's blog. I was planning on doing this for 3 weeks total and then switch over to different types of intervals. Wednesday is a 5 sets of 3 minute sprint/3 minute recovery. Friday is 3 sets of 5 reps of 20 second sprint/ 40 second recovery. This is one set of 3x20/40.