You can make amazing progress in 20 years in real estate as a side investment. Not 'get rich quick' by any means, but better than just a job and a 401k that take +/- 40 years. Altough running a real estate portfolio is fairly passive, it takes a lot of work to find and acquire properties. Less work than a business, but more work than a 401k for sure.
People who make a lot of money in real estate are ones that turn it into a business and invest other people's money as a general partner. This makes a lot more money, but is not at all passive. It's a full-time endeavor.