I'm a cyclist, 100km without much effort, but a year ago I started running and it was super hard. The muscles are used differently, the body get impacted with every step and it takes some learning to find the right speed to reach whatever your target is. Pain kills the fun, the only way to succeed is to start slow and on short distances.

My goal was strengthening hip (one side has a three plates and a few screws in it) and knee mobility, with losing weight being an extra that I only reach this spring due to diet changes. I did find my Zone 2, where I can chat with a running buddy, and slowly extended the distances I run: Now 5km at 5:40m/km are the norm and 10km is just as possible. Injury-free so far and no ambition for a Marathon. The thing I now have to work at is the cadence which I (at 193cm) had trouble with on the bike as well, but shorter strides at a higher cadence are important to prevent knee injuries.

Whatever you do, I think sticking to it at least three times a week so you look forward for the next run, feel well during it and keep healthy. This is more important than beating records or running insane distances. Those goals are for those with the right genetics.