Thanks for all your good tips, suggestions and wishes!
For those interested, the event is called "Horizontale Jena" (the web page is in German, only). We walk through the hills one time around the German city Jena. So there are parts of the way which are relatively flat, but we have to go up and down the hills several times. I expect forest trails where you have to watch your step because of roots and stones, but nothing which requires special skills or heavy boots.
The last couple of days I did some walking to see where I am right now. Friday 9 kms in flat terrain, I needed about 2 hours (inkl. shopping for eggs at my local farmer). I wore my new Trail Gloves, and after that my feet and legs hurt a bit. Saturday 11,5 kms in hilly terrain. 2:45 hours, inkl searching for one Geocache. My feet felt better and get used to the new shoes, and I was a little bit tired but not as much as friday. Later in the evening I did another short 3.5 kms without problems. Yesterday 4.2 kms in easy (but not completely flat) terrain, 45 mins. A good speed, but I doubt I can keep it for much more than 2 hours. And I developed a heavy soreness in my glutes. Now I know which muscle I need to work on
I will build up my walking distance each week. 5 kms more each week and I will have done at least one 50 kms walk before the event. I guess sprinting for endurance and strength training are more important, but knowing that I can do 50 kms without much problems will help me with the mental parts of this challenge. I read a few articles from people who did the walk in former years, and after 50-60 kms all that counts is your willpower and stamina...
Plus I can figure out how and what to eat on long distances. I have a 2 l bottle similar to a camelbak, and I am planning to bring a 0.5 l Nalgene bottle to make iskiate (water mixed with chia seeds, lime and raw cane sugar). I want to bring my own energy bars (similar to the recipe Mark posted in his blog), but I have to figure out if my body can handle them after hours of walking.
Does anyone has a suggestion how to handle the breaks? I read that some people just grab something to eat and drink and walk on. Sitting down on the provided benches seems like a not so good idea. Laying down for 10 minutes with the feet elevated feels like the right thing to do, but I am not sure.