I went through marine boot camp back in the 80's, so I can answer your questions.
First, the gas chamber isn't that bad. Your lungs, nose, throat, skin, and eyes will burn and you'll be shooting snot out of your nose like a fountain, but you won't puke. Nobody in my platoon did.
As for push ups, from what I recall, you don't do many of them like you see in the movies but I would try my hardest to improve your push ups skills as best as possible.
After the day is done, you get about an hour of liberty in which you can take as many dumps as you want but if you need to go before, you ask your drill instructor by saying "For a head call sir". They may dog you but they won't ever say no.
MRE's usually have about 3 to 4 items and they are quite filling. But remember, in boot camp, you eat for nourishment only, not enjoyment. So get used to eating fast.
On sundays, most guys will either go to church or polish their boots or write letters home. It's your free time but most go to church to get away from the barracks and drill instructors because if you don't go to church, you must remain in the barracks.
I won't lie to you. Boot camp is tough especially the first few weeks. You'll swear that your drill instructors are demons straight from the pits of hell and you'll hate them with every ounce of your soul. It's been over 20 years and I still hate my drill instructors.
Just remember that their goal isn't to torture you but to save your life. Whatever you do, just play the game. Don't try to be defensive or belligerent and don't talk back. Let them treat you like dog meat. Let them shout in your face. That's their job.
Just keep it in mind that boot camp is a very small part of the whole picture and you will get through it.