No reason to do 13 full lengths! Do about half that and you'll be just fine. Are you self studying? Kaplan's timing strategy is basically to give yourself 1m per discrete question and do all those first; then give yourself like, 6.5m per passage (reading + questions) but time passages as a pair; then give yourself like, 1-1.5m at the end to fill in answers for the questions you didn't answer.
Always select an answer. Don't second guess yourself. Move fast. Don't let your performance on a previous passage affect your performance on the next one. Force yourself to finish the passage pair on time.
Pacing is huge. That, and the other Kaplan strategies, were definitely worth paying for in my opinion. The books and flash cards were fine--but nothing special. The online resources, discipline, and strategy's what you pay for!
I found AAMC to be a joke compared to the real thing, and Kaplan to be closer but for the most part easier than the real thing too. My real thing had much more immuno/genetics/molecular bio on it, much more theoretical physics, and much less organic than I was expecting--for what it's worth.
(Edit: the AAMC tests that are available online for free are the only AAMC tests I took, but I assume that's what you're talking about. I found those to be much more general bio and physiology focused than the real thing, which felt like I was reading straight out of a research journal I had never seen before)