Marathon job interview tips?

On a personal note, I'd say that: yes, it's possible to be too energetic or over-caffienated.  There's a line between enthusiastic and spazz-out!  (And I don't think the line is necessarily "fine" in nature.)  I'd not heard of these packets before but if you're talking about the Roctane stuff, that has 35 mg caffiene per packet which is equivalent to a Diet Pepsi which is what I drink. I consume three cans of Diet Pepsi over the course of a work day and that's just about right for me - but that also includes food.  If you pound GU without eating anything, you're likely to get hyper and/or jittery.

Right or wrong, I always bring a purse. The smallest, least obtrusive I can. (I have reading glasses and I like to have my own pens and gum on-hand for breath freshening.) Just don't go for a clutch or tiny evening bag that looks too formal. Also try to avoid handbags - the ones that require you to use a hand to carry them. A shoulder bag would be best so that it's out of the way but it shouldn't be so lightweight that it flies around when you're on the move, nor should it weigh a ton.

I'm guessing you've listened to the cast about lunch interview. Stick to a soft-drink and be sure to compensate for any GU packets you've consumed. (Go for water or caffiene free things like lemonade or 7-Up.) Try to avoid things that are messy (pasta) and/or will require you to eat with your hands (hamburger, fries.) Eat strategically - when he/they talk, take a bit & chew quickly. I'm guessing you'll notice that when you talk, he/they eat. Just take turns and get some food into your system.

And with a bathroom break, do what you can to energize yourself naturally. I don't recommend doing push-ups in a public restroom (ew!) but some quick squats, stretching, etc. would probably help get the juices flowing.

I've been a lot of marathon interviews (the longest was ~13 hours over 2 days, including dinner, breakfast and lunch interviews, plus public presentations).  When I had my first marathon interview, a colleague recommended to me to keep a check list to review before session of the main points I wanted to make. For an energy boost he recommended eating a mandarin orange between sessions. 

Make sure you stay hydrated. I would not recommend having more caffiene, etc... than you are usually accustomed to. If they've planned the day well you will have a break between each interview session. Take every opportunity you can to walk to the rest room. There you can stretch, etc... to get refreshed, as Ashdenver mentioned above. Even if you don't use the toilet wash your hands, a little cool water can be very refreshing. Take advantage of having a few moments of privacy. Then make sure you get back to your next session a few minutes early. 

Before the interviews start completely turn off your phone (don't just put it on vibrate). Put an away message in your email if needed. Make sure family, etc... know in advance that you will not be available for the entire day. During your breaks, do not check voice mail, email or texts. It will only distract you. 

I recommend having a separate note card prepared for each interview session. On one side of the card have the questions you want to ask written out, so that you can review them for the upcoming session.  Some questions you'll want to repeat from session to session, but make sure you have others tailored to whomever you are meeting with. Afterwards while you are on your break, use the reverse side of the card to jot down the names of the people you met with and a key point about each that came up in the interview. You'll use these to help personalize your thank you notes when you get home. 

I also take a note card to review with two lists: 1) why I am interested in the position and 2) what about my background makes me the best candidate. I've found this really helpful to refresh in my mind the aspects of my experience and accomplishments that I will want to highlight in each session. 

Be prepared to answer the same questions over and over. The career tools interview series preparations are very helpful for this. Don't worry that you sound repetitive. Even if one or more people sit in on multiple sessions and hear you say the same thing repeatedly. You'll want to tailor your answers slightly to your audience in each session, but be consistent regardless. 

Often you will meet with the most important person last. When you have the least amount of energy. When you are quite tired of hearing yourself talk. This is when the notecards, etc... are most needed to remind you what is important to do, say and focus on. 

I usually bring a purse and a slightly larger bag in which I carry a padfolio and a bottle of water. Some organizations I've interviewed with did not offer any beverages or offered far less than I needed to stay hydrated through the day. In my purse I also bring tissues, aspirin, bandaids, gum, cough drops or other small items that will allow me to get through the day comfortably. What I bring varies depending on the climate and time of year; the purpose is to make sure that you can take care of any personal needs that may come up during the day as unobtrusively as possible. 

At the end of the day, send a thank you email to the hiring manager and/or whomever escorted you throughout the day. Then start writing your handwritten thank you notes to everyone you met with, as well as anyone you sent the thank you email to. 

Great comments on this thread. I'd just add: bring protein snacks in your bag such as nuts, or even a pre-peeled boiled egg. I did this for my last marathon interview and, when necessary, ate a few nuts in the restroom. Your body will burn protein more steadily, over a longer period of time.