A number of members found the Kaplan CAT's too difficult and not very representative of the actual test. However, Kaplan recently changed its CAT's. Apparently, they are easier now. I don't know, however, whether this necessarily means that they are now more similar to the GMAT or not.
It's better to trust the GMATPrep score, as it is the best indicator of how you will perform on the GMAT. A distant second are the Manhattan tests.
The best way to strengthen one's test endurance and concentration is to fully learn the fundamental concepts, practice with GMAT-like questions under timed conditions, and use CAT's. For your CAT sessions, make sure to simulate real test conditions as much as possible. That means taking the test under timed conditions and doing the AWA. Also, take the alloted breaks in between sections.
Concerning the breaks, read
this discussion. Food should help increase your energy and concentration.
And, just in case you haven't seen this yet, here's a helpful link on CAT practice:
all-gmat-cat-practice-tests-links-prices-reviews-77460.html