There's really no way to make a heavy bike lighter in any significant way. Unless you are going up a lot of hills during those rides, the weight is not holding you back as much as you think anyway. Your idea about tires is good. It will reduce rolling resistance, and if the wheels end up lighter, your bike will feel snappier. I would not go narrower than 1.5 inch slick tires though. After that, the next thing you can do is to deal with your handlebars. Get them to the level of the saddle, and put bar ends on them so you can ride a bit more stretched out every once in a while. If your bike has motorcycle style riser bars (as I think it does), swap those out for a flat bar (and then add the bar ends). Cut the flat bars down to shoulder width. Most of these bikes have handlebars that are so wide and high, you become your own braking parachute. Do whatever you need to do, see whomever you need to see, to get your saddle in a good position. This makes a big difference in the power you can apply to the pedals. I recommend not too low but also not too high, and knee-over-pedals that puts your knee slightly behind the pedal spindle. Other than that, gradually increase your cadence to at least 60 rpm in a way that you can sustain without getting too short of breath or anything like that. This will allow you to go farther and longer with less effort and fatigue (do this gradually).

But I would not spend too much to convert anything. Better save the money for the newer, more performance-oriented bike you will probably want later on.