Hmm, i've never actually used my iPhone for it, always used my iPod Touch, so not sure what the problem is there.
I'm at the stage now where my only real boundary is time constraints. I'll generally run 11km at a time, which usually takes about an hour or so. I've got a pretty good level of fitness now that I don't really get 'tired' anymore. Running at night is actually proven to be better for you as it's colder, your body uses more energy to heat itself therefore you expend more. I always prefer it too as I don't have to worry about dodging traffic.
http://nikerunning.nike.com/nikeos/p/nikeplus/en_GB/plus/#//runs/history/228118290/
That's my Nike+ profile if you fancy a look.