I just started my outside running this year by running a measly 15 minutes in the dark. It was difficult. Number one, it's difficult running in the dark, but my new work schedule makes it impractical to run in the afternoons--especially since within just a couple of months it will be the hottest time of the year. Number two, it's different running on pavement/concrete, than it is running on a treadmill. Instead of pacing yourself by the steady rolling of a belt, you need to pace yourself by feeling your breathing and your heart rate. If you're not used to it, it takes time.
So, it was hard, but not that hard, and if I'd have allowed myself more time after writing to run, I could've gone longer. I'm not sure what pace I was running--likely ten minute miles.
My advice is this: if you are beginning an exercise program, or changing one up, take it slow at first. Work your way into it for a few days. Adjust.
It's important to exercise, but not at the expense of injury due to too much enthusiasm and biting off more than you can chew.
Now tomorrow, I will go longer, and stick to streets where the streetlamps are working, but I will begin slowly and build up speed as my body feels like moving. That's how it's done.