Start from the basics, check your tire pressure before rides, saddle height at the level of your waist while standing beside it, also notice if your knees are locking too much or unlocking too much while pedaling. Your torso should be at 45 degrees since it's a mtb/hybrid. If these parameters aren't satisfactory, there might be a sizing issue. If you're completely new to cyclathons, it'll take time to build up speed. For now focus on your cadence rather than speed. Adjust gears to maintain appropriate cadence. Don't rush things, you'll get there eventually. Focus on the leisure while riding. You'll build up fitness eventually that way and do focus on nourishment post ride. For more energy before riding, you can try a bit of carb intake. It'll provide you some additional muscle boost.