Believe it or not, my favorite workout is intervals, no matter if it is 400m intervals or 2km intervals, I just love it.
In brief, intervals are short, intense efforts followed by equal or slightly longer recovery time. One of my favorite programs is 400m intervals: after warm-up, run 400m at hard effort (e.g. 1min 30secs), then rest for 1min 30secs, repeat for 12 to 15 times.
I like the feeling that heart rate is constantly rising and falling during the workout. I like the bursts of sweat which help to get me into better cardiovascular shape. While the process is often “painful”, I like the natural “runners’ high” created and feeling energetic and happy afterwards.
Of course, interval training cannot be done too often. Usually once a week is very sufficient for me. Also, it is very important to have sufficient warm up before that and ample cooling afterwards. Otherwise, it would be easier to get injuries.
I do believe in the motto: no pain, no gain!