How to train for an 8 km run as soon as possible?

Running will actually help you rest more effectively, if you exercise, you’ll sleep smoother, and feel more awake in the hours that you are, well, awake. All you can do, is practice, by which I mean train. Go running right now and run your absolute max distance (I’m guessing would be 3-4km). Work on that, don’t push yourself to run 8k before you are ready. The adrenaline on the day will be enough to push you 1-1.5km over your usual. Try running 3km every second day. Every other day, you could lift small weights and do something like yoga to give your body a physical rest, but keep your metabolism up and your energy up.

Once you have a schedule locked in, and you know your distance limit, try experimenting with energy drinks/bars to see if there is anything on the market that really boosts your energy and makes you run like 2km over your usual limit!

Best of luck, even if you don’t finish the 8k, or don’t finish in a marathon time, know that the charity still gets the help, and you have spent a month getting really fit! Maybe use this as a practice marathon, you could find another one in your area to do in, say, 3 months and be better prepared :)