You'll be fine. Just be aware the altitude will get you, likely on the 2nd day. Drink a decent amount of bottled water when you're walking around as you'll also get a bit dehydrated.
Lived there in '93-'94 so don't have any current recs, but I enjoyed my time there. I'm a bit of a foodie and the food is mostly fantastic. At least one morning, get breakfast at a Sanborn's. Even though it's a chain, it's kinda like nothing in the US. Try the chilaquiles. Maybe go to the one near the Zocalo, which is worth a visit. Otherwise, the sushi is surprisingly great in the nicer neighborhoods, especially unagi and snapper. Argentine steakhouses are good bargains. I even tried BBQ bull testicles at one (the sauce was everything). For a real old school Mexican dinner, San Angel Inn is a classic and close to the Diego Rivera/Frida Kahlo house/studio.
If you're not a picky eater, try some different stuff. Best way to do that is at a tacqueria (or a street taco stand), where you'll find most of the tacos don't resemble what you get in the US. I first had tongue (lengua) there and loved it. Also had fried grasshopper (chapulines) which were surprisingly tasty. Fried maguey worms taste like potato sticks. Most of the fresh fruit you'll find is perfectly safe to eat (except maybe strawberries from a farm stand). Most of the food is not all that spicy, but if you get anything in a chipotle sauce (octopus in chipotle was one of my faves), it will pack a bit of heat.
Polanco is a good neighborhood to walk around. It's the fancy/schmantzy part of town, but also a good amount of folks shopping at vegetable stands, having coffee at sidewalk cafes, and window shopping. There's no big attraction there, it's just kinda charming and very safe. La Condesa is also a nice neighborhood to spend time in, with beautiful old buildings and a very European vibe. I hear the adjacent Roma Norte colonia is also quite nice these days with gardens, shops and cafes (it was pretty much all residential during my time). The Zona Rosa was the big deal nightlife area when I lived there, but not sure it's still as vibrant or safe as it used to be. A lot changes over 30 years, so anything I say here, you should double check to see if it's still true.
If I were to only do one big museum, it would be Museo Nacional de Antropologia. Lots of cool stuff to see including Mesoamerican artifacts. If that's not your thing, the big art museum Bella Artes in nearby.
If it doesn't gross you out and you really want to hang with locals, there might be a bullfight going on. If nothing else you'll have experienced something unique and the beer is cheap.
Enjoy your trip!
