Both on a day-to-day level, and in a crisis?
I'm married with two DC. Recently we've had the latter situation; I went to the out of hours GP with a shocking headache, and ended up 4 days in hospital with meningitis. We have no family locally who could look after the DC. It crossed my mind several times that if DH hadn't been there, what would have happened to my kids while I was comatose on a drip?
And just the day-to-day stuff; my older DS is a right handful and after a day trying to manage him and spend some time with younger DD too, I'm often weeping with tiredness and frustration. Knowing DH is going to be home to help with (nightmare) bedtime and then all the night wakings is what keeps me sane.
So seriously - how the hell do lone parents cope? You're all heroes as far as I'm concerned!