Bass is nothing but snare drum mounted sideways, and tenors is matched grip spread across 5-6 drums. Your left hand will pick up by playing bass, and it will get there with some serious practice! The more you immerse yourself into percussion, the more your hands and mind will mature, so give it some time!
Getting in shape you say? Core, balance, endurance, cardio. The main four things you'll need for marching. Then again, you have marched, so you know what it takes! No need to worry too much (with the exception of larger drums like 28-32's). Just stay in shape, and learn to always be relaxed. In fact:
http://www.drumlines.org/threads/ubbthre...html#Post206093http://www.drumlines.org/threads/ubbthre...html#Post206092Just two topics that are still floating about. There are quite a few more on here, and the search bar will be your best buddy! However you go about doing it, ALWAYS STRETCH you muscles. Whenever you pick up a pair of sticks/mallets/drum to prevent injury and strain. Most importantly, have fun!