I wasn’t physically active before I started running last summer. I wasn’t out of shape really, but I’d run a 5k in 30 mins, feel like I was going to die during and then struggle to walk for a day or two after.
Took about 3 or 4 months to be able to run a 10k and gradually upped my long run distance most weeks. Once you’ve got a decent aerobic base, the legs tend to catch up and adding distance isn’t too difficult, I think.