Since I am prior service, is it true that I only need to attend a 2-week course for Infantry? If so, how can I better prepare?

Be ready to start walking. If you can get to your armory during the day see if they'll help give you a refresher on some things. Dig into the regs and manuals to start building the knowledge, even if don't know where to apply it yet. If the unit tells you that you still have a way off (due to when they could trap a school seat) and are still drilling with them, watch everyone else.
Each RTI might have a different daily schedule but expect walking, weapons handling and familiarization, walking, land nav, walking, drills, more walking, a STX, and to finish it off a 12 mile walk. Once you find out to which RTI you will attend, you can direct specific questions to that location if your unit cannot answer them.
They normally break the classes down to have the people going through to make it resemble an organic platoon (E5s as TL, E6s as SL, etc) and will place you accordingly within.

That is true. Once you get assigned to your unit in SC ARNG, your company will submit an ATRRS application for for an 11B MOS-Transition course, it is two weeks long.
You need to prepare to pass an APFT and a road march at that course.
You also need to create a new DTS account in the Reserve components and get a new travel card because if you had a card on active duty, it was probably cancelled.