I started running about a year ago, I was overweight and in a far worse shape than I thought. Because I used to cycle reasonable distances I assumed I would be able to run too. Being out of breath after trying to catch the bus should have been indication enough that it wasn't the case. Basically all I could do when I started was half a km. I still remember the first full km I ran within maybe a couple of weeks of starting to run. To be honest it was probably the biggest and hardest milestone, 2k and 5k didn't feel as special. After the 5k mark I tried running for an hour without going for any particular distance, ended up with 8.6km and basically I knew 10km was within reach. First time I ran 12km was because I pretty much got lost, took a wrong turn and ended up in the parking lot of a ferry terminal. Ran 15km recently and I've signed up for a full marathon towards the end of the year, I know it will be hard, but there's no other milestone to shoot for anymore really.
From my experience, if you can run 2km, you can run 5, and anything up to 10 is just more of the same, if you pace yourself. Don't try to go for speed at first, just find a comfortable pace you can stick to (I found a heart rate monitor helps me quite a lot with that). Consider improving the pace after you have the distance firmly in the bag. For example at the moment I have days when I run and go for distance but don't really look at the time, and days when I try to go for a new best over 5km.
Above 10km pacing is critical, and the mechanics of running become more important, posture, technique, not getting your joints hurt, hydration, dealing with chafing, and really importantly, recovery time. I find that running on the day after a 10km+ run ends up hurting a lot more on the next day, so I now avoid it altogether. Looking forward to "hitting the wall" now, and all that wonderful stuff.
