I started in the morning at the GR5A pole near the viewing point at De Panne. It was around 12°C and cloudy. First few kilometers in the forest and then into the dunes of Koksijde and Oostduinkerke. Running was possible, but I walked some parts to save energy. Most of the dunes were done at kilometre 15, where I had cached a bag with water and some food. Approximately every 15km I hid a cache in the bushes so I didn't have to go in shops along the way. After that, in Nieuwpoort you hit the dyke and there was a lot of running on hardened surface, just until Nieuwpoort-stad, where a grassy path starts along the canal at the Rattevallebrug, where, again I continued on a long stretch of asfalt towards Middelkerke and then a hardened dune path until km 30 where I hid my second bag of food and water. At that point I still felt good and was enjoying the run. From Middelkerke the GR goes through the dunes to Oostende passing domein Raversyde, a complex of bunkers from the second world war and passing the Ensor church back onto the dyke. Again, few kilometers on the dyke, when passing the Venetiaanse gaanderijen, I took a left towards the Maria Hendrikapark. Through the park and onto the ferry. I was lucky, it was just filling up when I arrived. Five minutes after I boarded, it left. It could get really cold when you have to wait for the ferry, especially in winter, so I would suggest at least taking a windshell. After unboarding the ferry, there was my third bag and I took time eating and drinking along the coastal path towards Bredene. At Bredene, there was a paper about a new/alternative route of the GR5A which took you more inland instead of over the beach. It looked as if there was some construction going on at the entrances of the beach, which is why the route was changed, I guess. Past Bredene, back into the dunes, with loose sand, which made running at some parts very hard. Luckily after that, it changed to forest path and after a passage through De Haan, only a few kilometers to the finish, the wooden GR-pole in Wenduine.
I edited my track from the GR-pole in De Panne to the GR-pole in Wenduine. Overall, route was marked ok, except for the dunes, where I used the GPX on my watch, which I downloaded from routeyou, often. Though the marked route has changed at some points compared to the Routeyou GPX (notably through the centre of De Haan), I think it is accurate 98% of the time.
There are a lot of shops along the route, which you can use for food, but there are a lot of places where you can hide some food and water beforehand too, if you have time to prepare.