Been there done that man.
Although I was working 45-50 a week the principle is still the same. I worked in a sawmill I'm not sure what you're doing. During the school year things can really suck. Your legs will feel heavy and stiff at the end of your shift. I ran 85-90 on singles during this time. It won't be that brutal but some days will suck. Understand this.
What I did since work started early for me was I took a nap for about 90 minutes or so after work. Set the alarm and got up and ran. I just could not find a way to drag myself outta bed before work to run.
It will be very hard to make large improvements in terms of PRs early on. I would recommend doing you workouts on your off days, they'll be a waste otherwise. Get lots of sleep obviously. If I did it again I would switch to 6 day a week training schedule if your job is really tough. That rest day has helped me a lot post collegiately.
You can make great improvements in base fitness though. Just by getting the miles in and working hard labor you'll become very robust, physically and mentally.
Basically.
-Try to get your mileage in 6 days
-Look at this as sort of period of base training at first while you're figuring things out.
-Get all the sleep you can
-Try to workout on your days off.
-Keep your workouts in control, if you're doing any, go overboard and you'll find yourself in a hole you can't get out of.
-Make sure too log your miles nice and easy, hammering WILL fry you
-Make sure you stay hydrated, you'll need to drink like a fish
-Food is your friend.
-Try to get into a schedule, doing your runs at around the same time every day, this helps you ration your time.
-You can't control the weather just dress for the conditions and you'll be fine
-Most importantly don't let this be an excuse, you have to find a way to keep plugging away, eventually adapt and be much better for it.
Hope this is helpful. Good luck.