I would eat about 45 mins before training as empty stomach cardio is counterproductive for certain people. Got lots to do with glycogen levels fat storage etc etc. considering you haven't eaten for maybe 8 hours your glycogen will be massively depleted and then you hit a empty stomach cardio session the first and easiest energy store the body will reap is muscle. If your planning on putting on and keeping some muscular size eat before cardio to level out the glycogen stores.