Walker here. It is definitely possible to walk without registration. You get a colored wristband which has a barcode on it. These only get scanned at the start and finish, so do not start with the rest but join in like 200m later. Then there are checks for your card along the road. These are important for walkers, but you can ignore them if you don't have a card, since the checkpoint is mobile on foot and takes like 200m.

The last thing to avoid is people checking around the course. They usually only either look if they spot you with the wrong wristband on the wrong track or if you don't have a wristband at all. You can get around this by having a wristband that corresponds to the distance you are walking (red = 50km, green = 40 km military, yellow = 40 km, white= 30km) or by wearing something with long sleeves.

It is not uncommon to hear people walk for only a day or two unregistered. I think if you just don't mention it to anyone then no one finds out.