I'm currently in AIT training as a 13B. It isn't that physically demanding in a training environment. The job comes with some high stuff. I'm loving it so far. All my Sergeants have said that our MOS is very often used for jobs outside of artillery such as infantry and MP duties. Mind you, this is second hand information. I do know that it is a high precision job. Little mistakes in your section could have bad massive consequences. So if you don't like getting chewed out for little things, look elsewhere? I guess all I can really say is that it has been good training. Look at videos on youtube and read up on Howitzers. Hopefully some prior service guys hop on and have some better info!
As far as hard work, it really depends on the gun system your battery has. 105mm are pretty easy but 155mm can be hard work especially if you're firing quite a bit. You do have to pay attention as little variances can send the round off by quite a bit.
FA are used as MPs or other functions. A lot will depend on your battery and how much/kind of training they do.