I use a rower to do 40-20 intervals - not as far as a bike to fall off!!! I start with a 2k warm up ( not hard going, about 10 minures) and then kick into intervals. I can do 10-15 intervals all out and I'm absolutely broken afterwards. I'd say if you're doing a 50 minute session of all out intervals you've either got a rediculously fast recovery rate or you need to go all out a bit harder