From my many years in Germany and an occasional volksmarch.....I'll offer you some tips and advice.
First, they typically run about ten kilometers. There's been a trend to offer up a twenty km volksmarch, but I wouldn't advise you to go beyond the ten range unless you've done it a dozen times.
Second, there are volksmarchs all over the countryside, from small towns to large ones. Some walks go through urban areas, but most are in rural areas. They can be on a all-flat surface, although hilly routes might be added for the "thrill-seekers". The trails are typically a well developed surface or even a paved trail.
Third, weather plays a key part in volksmarchs. If the temperature is up above thirty Celsius....I'd advise a morning march only. If you want to march in the heat....carry liquids with you. Yes, folks can get dehydrated and get the ambulance called out if they start to feel faint. It's not a pretty picture if you get picked up.
Fourth, there are volksmarchs all throughout the year....even in December. So wear the appropriate clothing.
Fifth, every single volksmarch has a fest tent set up and beverages offered...especially beer. My advice is to skip the tent upon arrival and only attend it after you complete the walk. If you are dehydrated....beer is NOT the best liquid to drink. Neither is wine. You ought to know better on that topic.
Sixth, there's a medal or trophy given at the conclusion...hence the reason why you pay some Euro to enter the walk. So collect your item at the end of the walk.
Seventh, there are stations....typically every two km throughout the walk with water available. If you think you need it....then get a cup and sip it.
Eighth. Yes, there are volksswims. They are fairly rare, but you can occasionally find a case where a village offers it as part of their big weekend.
Ninth, the stamp in the book business. Some guys are fairly serious about this, so they start up a book. They note their march that day, and get a stamp in their record book at the end of the walk. These are typically guys who do twenty to forty walks a year....for their entire life.
Tenth, it's supposed to be fun and simply a day out in the sunshine. It's not a race....so don't show up and pretend to set a strong pace.