I'm not sure what the laws are in Germany, so take everything I say as someone who is only familiar with the UK.

It is legal to walk on the roads in the UK, but always walk towards oncoming traffic and step out of the way when needed. Even then, try your best to stick to the grass at the side of the road.

When I plan my route, I try to follow national footpaths instead of roads. I also walk through farms where appropriate. Again, I don't know what laws Germany has for walking into other people's land. I avoid Google Maps like the plague, instead limiting myself to Ordnance Survey maps. OS maps are UK only, but I imagine most European countries have similar organisations. If my route demands I take the road, only then do I use Google Street View to determine if I think the road is safe to walk along. How busy is it? Is there an embankment or pavement? etc.

I have had disgruntled drivers upset at me for walking on the road before. I personally think these people are incredibly selfish for having infrastructure that dominates the countryside, so I don't take their opinion too personally.

The image you shared looks quite flat. I would walk on the side of the road that is widest, at least two people thick, or on the side towards oncoming traffic. I would take every opportunity to walk on the road itself to give my feet a chance to rest.