How far/long are you willing to ride the days prior to the Sportive?
The region around Bruges is pretty flat so I think you'll have to do quite a big ride to get to the hellingen. A few cobblestone sectors should be easier/closer to find.
Maybe you could hop on a train to Ieper and do a ride in the region around the Kemmelberg (and the other hellingen from Gent-Wevelgem). Should be a good preparation for the Sportive without doing the same hellingen.
Research the course: Check the route of the Ronde-sportive and identify the cobbled sections and hellingen that you want to train on. You can find this information on the event website or by consulting a map of the area.
Plan your route: Use a mapping tool or app to plan a route that takes you to the cobbled sections and hellingen you want to train on. You can also consult with local bike shops or cycling clubs for advice on good routes.
Check the conditions: Keep an eye on the weather forecast and check road conditions before heading out. Some cobbled sections may be difficult to ride on when wet, so plan accordingly.
Start slow: Take it easy on your first ride and gradually increase the difficulty as you get more comfortable on the cobbles and hellingen. It's important to ride within your limits and not push yourself too hard.
Stay safe: Wear appropriate gear, including a helmet and cycling gloves. Make sure your bike is in good condition and has suitable tires for the terrain. Observe traffic rules and stay alert for any hazards on the road.
Enjoy the ride: Remember that cycling is supposed to be fun! Take in the scenery and enjoy the challenge of riding on cobbles and hellingen. Good luck with your training and have a great time at the Ronde-sportive!