Not sure how to answer - I live in a city of 850,000 people. It's a 5 minute walk to a main road from our house. Main road has some stores and restaurants. Downtown as in with the skyscrapers would take hour to 90 minutes to walk I would think. Non peak hours its a 15 minute drive.
Well, I work in D.C and I am right across the street from the National Mall. I stay in Alexandria most of the time and there I am right off the main drag, King Street.
No center of my community where I live. I'm in the suburbs where nothing is centralized.
As for the center of the city, 13 miles. I try to avoid going inside the city limits unless I have to for a Doctor's appointment.
I live 11 miles from downtown, but that's not where you'd go to shop or eat out etc. Virtually everything I go to (office, grocery, kids' schools, eating out, church, doctors and dentists) is within 5 miles of my house.
I live in the downtown center of my city which is the 2nd largest city in the state. The downtown has a thriving nightlife and a significant amount of housings scattered throughout the office buildings and hotels.
Depends on what that means. I live in a suburban community and we have a couple of small "main drags" which I can walk to fairly easily. But then there are other cities and communities where there might be main drags. But it's pretty much decentralized in the suburbs. I guess Berkeley has a small downtown. Oakland has a bigger one, and of course San Francisco has a pretty big one.
But even then - the idea of a "city" is really more of a political entity and more or less a manmade construct. We of course have freedom of travel and around here the suburban areas all more or less blend into each other, where often the only apparent difference is the color of the street signs.