I do this quite a lot. Usually I just do 20 minutes and try to keep going 2000m per 10 minutes. But it's part of my longer routine wich has some other exercises of this kind (cycling, cycling with hands workout or this thing on which you walk veryfast - sorry I don't know names of those in English) so I cannot end up on the floor out of breath

I do 45-50 minutes of this kind of stuff before normal workout with weights and machines.