Disclaimer: I am not a very experienced runner, only about 15 races done but have learnt some things which can help first timers at any marathon. Also these are not prescriptive so use your own judgement and dont blindly follow these tips. Also assuming that you have done the hard word and training and you are all set for the race.
Firstly congratulations that you are entering the fabulous world of marathons (10K, 21 K and 42 K included).
These tips might make or break your race.
Go for bib collection: This is a very important aspect of the race. You get to meet and see fellow runners, you get to interact with experienced runners, get tips and also share your anxiety with others. You also get important information like parking, start time, start point venue details etc. You also get exposed to fancy equipment, latest launches from sports companies which is useful. (not to forget the cute guys you would see and can get acquainted with).
Make a checklist and follow it to the T: There are many small things that you need to remember, the bib, t-shirt, shorts, pins, breakfast, Vaseline, shoes, socks, GPS watch or phone, charging the phone and watch etc. etc. etc. You cannot remember everything. Make a list to make everything fool proof. You don’t want to be at the start line realising that the bib is at home.
Hydrate on race day but don’t guzzle: The hydration should be built over the week prior to the race and not on race day morning. Drink too much and you would be lining up at the porta pottys or on the side of the road or too shy to go thus impacting your performance.
Use Vaseline: Its important, very. Friction would beat the shit out of your inner thighs and your under and inner arms. Imagine hands and legs rubbing about 10000 times in a 10 k race and more for longer races. Use vasline generously before any race. Its a must.
Have breakfast: Do not go into the race on an empty stomach. Have a light breakfast. Juice, muffin and a banana should do it. Don’t over eat else you would puke.
Use tape generously: Tape all the parts where you get blisters usually and tape them up. All friction will happen with the tape and blisters will not happen. Nipples also tend to chafe depending on the fabric of you t-shirt. Its always advisable to tape them else it gets very painful.
Follow the plan: You have trained hard and you have a plan in mind. Don’t let it go by following some random runner. There will be thousands of people. Don’t go out too fast or too slow, don’t get carried away by the crowd. Follow your pace and stick to it like you have trained. (if its a cute guy or cute girl you are chasing then it might be worth throwing away your plan).
Keep hydrating: Its not brave to not drink. Your body tends to lose fluids while running long distances. Keep sipping at every water station or use some on your head to cool it down. Else you would get cramps or heat stroke. Keep sipping. Its ok to stop at or walk through water stations.
Smile for the camera and fellow runners: Believe me, it gives a high, an adrenaline rush and keeps you going. Cheer others, smile for all the cameramen, give high five. This will help you sail through the race.
Sprint the last 100 meters: I know you would be tired given its your first race but put that effort to sprint at the end, it would be ecstatic. That is where you are most likely to get clicked so also a good photo opportunity for your next profile pic.