Stress
Here comes the biggest headache on the job: the stress. Sure, power is great, and responsibility is cool, and arguing with people who disagree with you is...fabulous? All of that takes a lot of energy and focus. This person needs a certain thing, that group needs this other thing, and no matter who you help, someone is going to be upset. Oh, and if you do it either of those two ways, you remove a whole species of owl from its native habitat. Jerk.
There's also the campaign. The constant grind of kissing babies, shaking hands, and having to have opinions on whatever the big news story of the day is―"no, I don't think the governor should have said that"; "yes, Kanye's newest album is kind of a letdown"―can really wear you out over the course of a few grueling months. Make sure your willpower, stamina, and walking shoes are all up to the pressure.
While many of your decisions will have really low stakes, some of them will end up being life or death. Nowhere is this more evident than in emergency management and planning. Most municipalities have someone in charge of services during emergencies so that all the tools at the government's disposal, from police to search and rescue to government funding, are ready and available when they are needed.
Whether you're a mayor or a legislator, you'll have some degree of control over how the service operates and how much funding it receives. So when the storm comes and the people look to you for help, you better know what you're doing. Try not to stress out about that, okay?