I've been a PA for about 12 years now.
Don't be under any illusion, it's a hard job. The more people you work for, the harder it is. I wouldn't work for any more than two people these days as I've found that you're on a hiding to nothing otherwise. More than two and you're not really a PA IMO.
You need to be able to spin plates and constantly be thinking ahead. In terms of meetings, go through the diaries and find out which ones are recurring and what is needed for each one so you can get papers in on time without doing a headless chicken. If things don't come in on time then chase, chase, chase! You don't need to be a bitch but you do need to be assertive otherwise they will walk all over you.
Get access to everyone's calendar on Outlook if you can then pull them all up at once, choose a date and send a meeting invite. If you are setting up a meeting with external people then email them and ask them for their availability. Give them as much info as poss (i.e. 2 hours duration, at head office, covering xyz, involving Tom, Dick & Harry, etc.). When their replies come in, log the info on a spreadsheet, pick the best date for all and send them a meeting invite. Give them a day or so and if you haven't heard then again chase, chase, chase!
I don't have a to do list as such but I do use a system on calendar and email (which a bit too complicated to describe here!). I also do things routinely (i.e. order stationery on a set day) and use a bring forward. Also, as soon as something goes in the diary I start working on it - so if my boss is travelling I will block out the time in his diary, find out when he wants to go, put flights and hotels on hold, find out what he needs to take with him, start nagging people for reports and presentations, find out the contact/PA at that end, etc.
Also... don't be too helpful! You need to learn when to say no. You are in a difficult position in that you work for several people. That's fine if they're not too demanding but if you're not careful you will end up working your fingers to the bone. Don't be afraid of saying no. Terrible generalisation I know but some men at work can be incredibly lazy sometimes and if you find them asking you to do things you think they should be doing themselves then tell them it's not part of your remit (nicely of course!). People will have far more respect for you if you do.
I spend most of my time kicking people up the backside to get things done. You need the patience of a saint doing this job otherwise it will drive you around the twist...