I haven't in a while, but I worked my way up from a couple of laps to 2 miles continuously. Just do as much as you can, take a break at the wall if you need it, it WILL get easier.
Another thing to try is switching to breast stroke for a while if you run out of air. Sometimes a lap of breast stroke could keep me going for a few more laps of crawl. Backstroke could also work, but I'm absolutely terrible at backstroke.
I never really got the hang of flip turns.