Here is my update: Sat morning I ran 22.5km and on Sun morning 24km. I could feel the strain in my left leg, which by the way is (fortunately) not shin splints, because the pain is located about 10cm up from the inner ankle. I apply ice and keep it elevated as much as possible, but the rest of R.I.C.E. needs to be applied, too.
I had a plan of taking a week off in the end of January to rest before the last part of my preparation for Rome. Instead I will start this now with the following schedule: 3 days complete rest apart from biking and stairs to work and abs. 2-3 days rest with power walking/biking. Then a small run-test to see how my leg feels. Accordingly I will take up slowly running or continue cross-training for a second week. The biggest challenge in this schedule will be the mental exercise!
As every runner would, I am worried about my leg. But I am confident that with this plan I will be healthy to run in Rome. I can feel the strain in my leg when I walk, but it has not swollen and honestly I believe it is a muscle overtrained. If this is the case then R.I.C.E. should do wonders. Also I think it is important that I rest in time before it gets worse and turns into real injury.