Like you Amy I'm not looking forward to the nights drawing in - less because of dodgy folk on corners more because there's only one real road with both pavements & streetlights round us! Also we all tend to plateau out from time to time, my guess is the 2 things have just conspired together for you this time.
Don't worry about the fitness it'll come back ever so quickly when you find a new routine.
So what do you like doing, how long have you got to do it (no point aiming for 30 mins 4 times a week if in reality you are only ever going to make it once - you'll just depress yourself)?
All 3 of Nessie's points are spot on, you might need to change your routine slightly in the mornings or weekends but it'll be worth it.
The other point I'd make is why not use 5k 'races' as part of your training routine. I assume you don't enter them with the intention of winning, so why not enter them with the intention of adding them to your trainging routine. This should give you some milestones to work, the chance to run in safety, the buzz of joining others and some nice little medals/mugs/certificates/bottles of beer to admire on those long dark nights.
Keep smiling