Further (as opposed to "farther") refers to continuing a movement along a path or direction. "I want to run further" means, "I want to continue running [down this road], and not stop right now." This is the sense of your first statement, and would presumably be stated during the run itself. The opposite would mean something strange like, "I want to run less down this road than I have already run" or "I want to have run a smaller amount than I just ran."

As opposed to "I want to run farther", which refers to the overall measured distance run, and does not imply continuing. This is the usage that matches your second statement. This would be used probably while planning the run itself, not during or at the end of the run. The opposite of this might be, "I am low on energy, so I'd like to run less than usual." More likely, this would come out as, "I am low on energy, so I'd like to cut the run short."