Let's get real here. An 8 hour ultra for an average runner is going to be more like a long hike than an actual road race. If you try to run straight through the first 15-20 miles, you will be completely broken down by the fourth hour. You need to find out how much you can run and how much you need to walk over a long time period to still be able to move well and refuel as you go. Instead of doing your long runs based on miles, I would do them based on time. Start with 3 hours and try running about 75% of the time with walk breaks 25% of the time. That may be 15 min run 5 min walk or 8 min run 2 min walk. Also practice eating and drinking. Flat coke is essential for ultras. Get used to it. If 3 hours goes well, add another hour until you get up to six hours.
And for your runs during the week, focus more on time than distance. Try to find a pace that you can run for an hour without feeling tired. Then, add a slightly longer run of 1:15 or 1:30 mid week.
You are not going to get faster in four weeks. But you can do a lot to acclimate yourself to being able to keep moving for 8 hours.