For someone who doesn't run it's quite a way, there's no shame in running as far as you can and then stopping to catch your breath.. then get running again.
When i was in the marines we did lots of fast 1.5 mile runs to build up our base fitness, after a few weeks 10 miles was easy, then 10 weeks later we were covering 60 miles in a night, with 10st of kit on our backs after very little food for days!
Doesn't matter how you do it or how long it takes you... so long as your doing it and pushing yourself.
Body gets used to anything pretty quickly as long as you stick at it and as soon as you notice the fitness building up... it wont even hurt, you'll just feel stronger and better
