Big concepts to consider... Don't get hung up on a plan and get discouraged when something gets in the way. I thought I could hunker down and wipe the prerequisites out in three semesters, rail the MCAT, and soar into a school. It won't work this way or any other way your planning things. Now in my fourth year back to school on my second application cycle I can promise work will interfere with course selection, colleges aren't structured to offer multiple lab sciences simultaneously with much easy (lecture, lab, and sometimes recitation will overlap negatively), and if you have a family, sports, dates, etc. will further complicate things. I don't remember reading your age (many 25 year olds think their non-traditional students and won't have some of these concerns) but, if you do just accept that the path is harder for us and isn't travelled often for this reason. Keep your chin up and make whatever the best decisions are at each turn and if you're anything like me the more time and energy you expel on the journey will further ignite your passion to reach the finish.
As for a specific response to your question, I think Khan Academy is great. Wen you're learning concepts continually ask yourself why? Never memorize, in biology especially, everything is repeated elsewhere (I speak of enzymes and transporters that are used over and over accomplishing the same tasks for different reasons) and understanding why things are happening will take a little extra time but, it will give you droves of critical thinking ability to deduce future concepts on your own. Science is gloriously complex and graceful at the same time, appreciate it and the study habits will appear to you in ways you never imagined... Good luck.