Get your hiking boots ready. In fact, get about 6 pairs ready. This guy has calculated the longest possible walking route on the planet that excludes any use of boats etc to cross water courses.
At 14,334 miles you could complete it in about 3 years if you’re prepared to cover 20km every single day. You’ll clock up 123,000m (76 miles) in elevation change which is the equivalent of climbing Mt Everest 14 times. If that doesn’t scare you off then maybe the places you have to travel through will. Fun countries like Zimbabwe, Uganda (with the highest malaria rate in the world), South Sudan (the 3rd most dangerous country on the planet), the Sudan Desert, the Sahara Desert and then on to Syria, (the 2nd most dangerous country in the world) and the entire way across Siberia known for winter temperatures of -39C (-38.2F).
There’s no record of this ever being completed and I can think of about a thousand reasons why that makes perfect sense. So if you want to do something original, here it is.