I have been running for a long time, but it had always about 3 mile lengths, and never super long distances until 2 years ago. >6 miles was almost impossible. the key for me to run longer was to run my limit, hit the wall and keep going, and after a run a week for months, then I'm doing 8 mile runs, 10 mile runs, 20 mile runs. eventually, I hit a second wall: boredom. I don't think that super high endurance is very rewarding, at least not for me, and to get better at it requires a lot of pain, and even potential for injury (and then you can't train and you go back a few months). I also think the health rewards of runs reach an inflection point around 5km, especially after factoring risk of injury. if you can't do a 3km run, run 1, or 2. then wait ~48-72 hours, and run 2 again. eventually, you'll be able to run 3, I guarantee it. but what is a lot less certain is whether you'll enjoy it and feel rewarded. I've found the more endurance based an activity, the more mental it becomes, just as original comment was getting at. for some people, thats good because its the only way they can be competitive. for others who have more athletic talent, it may be frustrating because it takes more effort than most other exercise.