It is generally advisable to follow the manufacturer's guidelines regarding the first service of your motorcycle.
During the initial break-in period, it is recommended to avoid putting excessive stress on the engine and components. This is to allow the moving parts to settle in and for any potential manufacturing residues to be flushed out. Following the manufacturer's guidelines for the first service can help identify and address any issues that may have arisen during this initial period.
That being said, if the manufacturer's guidelines permit it and you feel confident in the condition and performance of your Royal Enfield Classic 350, a 300 km ride may be feasible before the first service. However, it's important to assess the risks and consider factors such as the condition of the bike, your riding style, and the terrain you'll be covering.
If you decide to undertake the long ride before the first service, it is advisable to be vigilant and listen for any unusual sounds or vibrations. Take breaks during the ride to allow the engine to cool down if necessary, and keep an eye on the bike's performance. Additionally, ensure you have adequate tools, safety gear, and contingency plans in case any issues arise during the journey.
In any case, I recommend consulting the owner's manual or contacting a Royal Enfield authorized service center to get precise recommendations for the first service of your Classic 350. They will have the most accurate information and can provide guidance based on your specific motorcycle model and circumstances.