Hi Ellen, I'm new too, and also run alone. I don't take a mobile with me, but I do take a high decibel alarm with a rip cord which you can cause a distraction with, if nothing else.
I tend to road (pavement) run on quite a large housing estate, so I never feel too isolated.
I've found it helps to vary your route, and the times of day to go out. I usually set some goals from the 5k training plan to work to, and as some of the others have said, having an ultimate goal, such as a race, really helps you stay motivated.
Good luck, H