I usually start on the speed bag for about 20-30min left left left right right right (and other variations. Then work on my heavy bag, usually start with about 10min of left left right, left left right (and switch off) then eventually the end of the routine is left, right left right for about 5min going as fast as I can. It will seem impossible if your just starting out but its a good feeling after you do it for about a month.