Tires make a huge difference. Having a comfortable position with the proper leg extension is also huge. The handlebar thing comes into play with position, although some Ergon type grips may allow for a flat bar without discomfort especially if you add mini barends to give your hands something to pull on with hills or simply change your hand position.
I also find SPD pedals with appropriate bike shoes to allow for a more efficient pedal action since I can pull up as well as push down. As it says in the Bible"be kind to your Ass for it bears you". A saddle that is too soft and rubs the inner thigh can really drain your energy. A slimmer harder saddle may add to distance traveled with less effort. All these are fairly cheap fixes.