@ remarks:

1. Interval training is aimed squarely at the cardiovacular system, ie oxygen transportation, the heart. So it is feasible to do it some other way because you are primarily stressing the heart to develop it.

2. Having said that, you will benefit more from doing them on the bike, because the muscle groups that are used for cycling will also develop.

As your fitness increases, endurance training will have to take over most of your training time.

I think it is important to understand interval training, so check this out: It is a great tech write-up without the eye-glazing jargon.
http://home.hia.no/~stephens/interval.htm