IMO I don't think kaplan tests are very accurate indicators for actual score. Kaplan CAT's have always given scores much lower than on the actual test. I've seen Kaplan score be off by 70+ points. That being said, they are pretty good for practice use though.

Given kaplan's terrible score reports, you may not need to make as huge of an improvement as you think. If you haven't done any studying at all, spend some time to familiarize yourself with the test format, work off any rust you might have on the math section and take a real diagnostic.

I recommend using GMAT prep software for this. By far the best indicator of how you will score on the actual GMAT. Prepare your study schedule depending on the outcome of this test.