well i have been cycling much father distances , say 20 - 30 miles regularly (only cycle so little cause the amount of work i have) and push myself just as hard as before, and i have no sickness or anything, then have a mini core workout, .... so i am guessing the sick feeling i used to get was due to my body getting all pumped only to stop right when adrenaline kicks in, and my body has warmed up to it.