until I learned the glories of xanax, and was able to get on a plane, I took amtrak to disney world lots of times. now I can fly and haven't taken it in a few years.
but i keep looking at the prices every time i price six (yes we need six) plane tickets, and think we may be going back to the train.
i also am originally from Philadelphia so that is where I left from, so made the exact same trip.
$118 is an excellent price, it is usually more than that.
there is an auto train from lorton virginia, but it is pretty expensive.
first- do not get off in orlando. get off in kissimmee, much closer and traffic much lighter. if you booked orlando, go back and see if you can change without much price difference. (like airlines, prices go up and down).
second, bring snacks, the cafe car is terrible. spring for the real dining car for your meals. they are actually decent. the cafe does sell adult beverages, partake, as sleeping in a chair is difficult.
third- use the bathroom early on and don't drink much toward the end of the trip because people are incredible pigs and you don't want to use the bathrooms towards the end.
fourth- bring plenty to do. assuming you have kids, it gets boring, most of the trip is at night in the dark, so nothing to look at. they claim they have wifi but usually has very spotty reception.
fifth- pack light as you need to lift your suitcase onto the rack over your head (unless you're lucky and get on the train early and get room at one end of the train or the other but i get nervous when it's not near me)
sixth- despite what i just said in number 5- bring pillows and blankets. the ones on the train are NOT adequate if you really hope to sleep.
seventh- don't plan on getting in on time, never happened any of the 50 or so times i've traveled long distances on amtrak. freight trains have the right of way, so any time there is a conflict, expect to sit and wait for your turn.
it's really not a bad way to go if you have the time to invest in it as you said. it is an experience if you've never done it.
enjoy the trip. if you have specific questions, let me know.