In my 30s, I ran regularly. My best 5k time was 18:08. That was about 20 years ago. I walk 10k steps everyday now but am not what I would consider aerobically fit. I want to start improving my fitness by biking, running, and other activity. In Sept 2023, I completed a parkrun 5k in 26:46 and then two weeks later in 24:02, after which I felt like I was going to pass out.

Running a 5k in 20:00 is a very big stretch. It shouldn't be impossible if I can commit to a program.

I may bet on this question for motivation and will post regular updates with my ongoing 5k times.