It seems like your pretty level headed about this but I want you to think of a few things. If you are at all competitive your probabbly gonna tear off down the trail on the start. Be mindful of the adrenaline rush....which will lead to arm pump in a very short amount of time (learned from experience). Stay cool and calm and you will be alright. You"re right about getting out of the way when the faster guy is coming, but don't get yourself into trouble trying to accomodate them. the faster riders are generally better and can find a way around you. If you have to get out of the way, pick a good spot and not into a ravine,swamp, bull bryers, etc.. Remember, if you can keep moving down the trail without having to stop and rest you will be amazed at how well you do. It wont take long before you start grabbing trophies, then the mad addiction has begun.
Good luck and have fun