The GR 20 is a 115 mi linear trail on the Island of Corsica. It's widely regarded as one of the toughest trails in Europe with more than 38,800 ft across it's distance. It generally takes between 10 and 15 days to complete the trail. The trail is often far from well defined. Often, you are simply navigating from one marker to another making it very difficult to get a good pace. There is lots of scrambling along the trail as well, with tough rocky climbs and descents. You need to arrive at the trail fit and ready for the challenge. Arriving at the trail unprepared will leave you with a bad experience at best, or injured at worst.
The GR20 is generally hiked from late June to early September, though the heat of August can make it an extra challenge that might be worth avoiding. The climate in the high mountains in the middle of the Mediterrane can be unpredictable, so you should be prepared with adequate rain gear, and even leaving yourself some spare days at the end of your trip in case you need to take a zero-day.