Although most people are back to normal in terms of getting around and doing things within a couple of days after returning from a long-duration space flight, there are long term effects which take an extended period of time for recovery and may never be fully eliminated. Here is a NASA report of a study that indicates that, despite a rigorous exercise regime (2.5 hours a day, 6 days a week), astronauts on long duration missions to the ISS lost around 11% of their hip mass during the flight. A year after return to Earth, they had not regained either normal bone structure or mass, which may take much longer (if ever). A typical astronaut on orbit loses as much bone mass in one month as an elderly woman does in a year.
This is obviously a problem for any mission contemplating long-term exposure to microgravity, and it isn’t the only one. Microgravity is also known to impair cardiovascular system performance, increase cranial and interocular pressure, and suppress immune response.
It is remarkable that in more than 50 years of human space flight, nobody has done any serious experimentation with artificial gravity produced by spinning spacecraft. We had a post here on 2022-10-09, “Artificial Gravity for Long-Duration Space Flight”, on this topic.
As I noted in that post, beyond microgravity, nobody knows what effects long-term exposure to lunar or Mars gravity will do to humans. A habitat in Earth orbit with artificial gravity would allow experimenting with this before establishing bases in those environments.