Most people will tell you it is necessary to take a pad, at least in cooler weather. The pad serves two purposes. Its main job is to insulate you from cold ground (the fill in the bottom of your sleeping bag will compress, and not add any warmth where you touch the ground.)Its second job is to provide comfort, so you don't wake up stiff and sore - assuming that you even get soundly asleep. (See how long you can lie on the sidewalk without anything under you.) How thick a pad you need is strictly a personal preference. Now, depending on how much padding you need to sleep comfortably, you may be able to make do with a closed-cell (non-inlfating) pad. Most people I know, though, want something a bit softer, which means you want a pad similar to what you bought.

No, the pad you bought is not light, by the standards of most folks on this forum. That doesn't mean it's a bad pad, though. It's about typical for a self-inflating 72 inch pad with a 3.5 - 4.0 R factor (which is a measure of how warm it will keep you on cold ground.)

One strategy for going lighter is to go thinner.The pad you got is 1.5" thick; they also make 1" pads that weigh less. (The 72" Thermarest Prolite weighs 16 ounces.) However, some people find them too thin to be comfortable.

Another strategy is to go shorter. I use a 47" pad (often called a "three-quarter length" even though it's really two-thirds length.) My lower legs hang off the end of the pad, which is not comfortable (it bends my knees wrong, since I sleep on my side.) It also means that my legs are going to get cold if the ground's cold. So, how can I use that shorter pad? Well, my pack has a padded framesheet, so I simply empty the pack and place it under my legs, which levels them with my knees and provides enough insulation to keep them warm. (Emptying it is pretty much automatic: after I pitch my tent, get out my sleeping pad and bag, set up my stove, hang my food, put my water bottle where I can reach it, and stuff my rain suit in a stuff sack to use for a pillow, the only things left in the pack are my first aid kit, map, compass, and water filter.)

I think you should go back to REI, with your empty pack, and look at the 1" thick, 47" long, Thermarest Prolite (not the 1.5" Prolite Plus) pads. Lie down on them, put your pack under your feet, and play with the amount of inflation. Lie there for several minutes, in the same position you usually sleep, and see if it's comfortable. If it is, trade the 72" Lite Core in on it - you'll save a full pound of weight and get a little money back to boot. If it isn't, stay with what you've got (or, possibly, trade it for the 48" length to save half a pound and $10.)

I've purposely refrained from mentioning a plethora of other pads, including Big Agnes, Exped, POE, and other Thermarest products. I didn't want to overwhelm you with choices and make this post any longer than it already is. However, if you have the time and inclination, look at all the other options, too - it's a great first step toward becoming a gearhead!