For time, you can just work out what it takes you to walk a mile and mulitply that by 13. Subtract a little for some running and there's your goal time. It's really hard to say what a "good" time is since we don't know if you'll be running 5% of it, 15% of it, 25% of it, and how fast you walk or run.
Don't train every day. At least, don't push yourself every day.
Perhaps set a weekly plan with 4 walks.
One is a speed work walk. You work on intervals, going faster, and pushing yourself.
Two are recovery/general fitness walks where you do basically whatever you're capable of at that time, these will increase as you get fitter.
The fourth is a long walk where you do no more than twice what the other walks are (you need to extend all of them as you get closer to 13 miles), but more than the same distance.
Eg to start it might be half an hour speed work, 1 mile, 1 mile, 1.5 miles. Then next week 30 mins, 1.2, 1.2, 1.8. And so on.
You can start with small increments now. By the time you're walking 8 miles you can probably pop bigger increases like 8 to 9 or 8 to 9.5.
Deb, in New Zealand