1st, find out how deep your 'frost line' really is. Here in southern Ontario it's '4 feet', so no way I need to dig down SIX feet for a water line. Less material to remove(and replace) ,the faster the job will be.
Assuming it's a straight line, point tractor's bucket to the 'end' point. Be sure wheels are straight,then start digging. lift bucket up a bit, raise the legs 6" then push yourself forward, lower legs and bucket, repeat until you get to the 'end'.
It is slow compared to an excavator but can be fun.
Please consider putting the line inside of 4" conductor pipe,weeping tile or conduit. This way WHEN it has to be replaced, it's a simple task and a LOT faster than digging up the trench again.