I've been lucky. The furthest is to the side of the road and in the shade, preferably. I started carrying a tube, a patch kit, and a pump as soon as I could afford them starting when I was 16ish. The only time I would have needed to walk over 15 miles back to the car on a bike trail was when my chain broke, but another cyclist stopped by with a chain breaker. Now I have an emergency kit containing two tubes, a CO2 kit, a patch kit, a multitool w/chain breaker, quick links, some cash, and a frame pump on each of my five bikes.