Please bear with me, as this might sound a tad like teaching a small child. I realize you are not a small child, since you can post complete sentences and march on a battery. This is, however, how my college professor teaches 4-mallets to percussionists in his studio. Kinda strange, so it's easy to remember.
Excess pressure hurts you, and hinders the speed at which you will learn the music. I'm sure that you have made a "gun" with your hand before. We've all played cops and robbers or made a shooting gesture at the end of a stupid hand shake. If you ignore that the outside mallets are sticking out of your hand, you can now bend the barrel. You
dont want to bend it too much though. Make your gun shoot the people beside you, not you. With your mallet in the center of your palm, it should now have the perfect place to rest with your thumb on top. Your hand should always look like you're correctly holding a gun, not like some thug would hold it. I would be lying, though, by saying that i hold my gun correctly at all times, or that I don't tend to shoot myself on small intervals. It's okay to deviate a tad, but you should try and engrave that into your mind.
My instructor made me go a couple of days without playing on bars. He had me play on a table 'til my hands felt better with holding all four mallets. This will give you a chance to get good double lateral (alternating strokes in one hand), single lateral (repeated single mallet strokes), and double verticals (both mallets playing together in one hand). While your working one the strokes, you can also work on spreading and thinning the intervals of the mallets. try and get the velocity out of your wrists, not your arms. Using too much arm causes you to downstroke more often, which slows down how easily you get to your next note. I personally do not believe in pulling the sound out of an instrument, but if you move from C to the G above it, downstroking would require you to go down, up, over, and down, as opposed to down, over in a slightly arch-ish fashion, and down. Less energy, better sounds, more fluid.
When we finally touched a marimba, we worked on holding intervals, usually in fifths or fourths (which is great help with Yellow After the Rain). Play 8 notes with all four mallets and then move up half a step and repeat (C G C G, C# G# C# G#, etc...) and come back down after your done going up. It's not only a great way to get comfortable with moving the mallets/enforcing a better technique, but its a good
warmup to start with even when you've advanced farther along.
This is stuff that became written into my mind by repetition. I realize you only have about a month, but this is a good way to start improving (at least, it was for me). If you can get the basics down, YAtR should be a breeze. Its a great beginner solo, though of the thirty people in my studio, a couple of us did not have to play it as our rite of passage. I hope this helps!