I do a different variation of HIIT training....
Get on an elliptical, start moving at the first level at a moderate pace (say like 55 rpms as an example), after 1 minute increase to the next level (keep the pace the same), after the next minute increase to the next level (keep the pace the same) and so on and so forth.... keep doing that until you can't go anymore
By the time you are done you'll be huffin and puffin like a mad man!
The most important thing is that you keep the rpm's the same (don't decrease), should start to sting at around 12-13 minutes.... I try to do 15 mins of that but I've been up to 20 mins, never could do more!
Its more of a gradual increase than actual "intervals" but its good because its like you're warming up before it starts getting hard!