I was a Category 2 mountain biker who hopped into a 5k once with no running training as an experiment. I ran in HS and college, but hadn't run in 14 years. I was slightly under 21 minutes, and it was the most excruciating 21 minutes of my life. I was sore for a week. About 3 years later, when I had transitioned back to running 100 percent, I was able to run mid 35 range.