I've previously used a very old, not maintained single speed bike, and after my father having recently brought a hybrid bike, it's been about 20 days since I've been riding it more or less daily, durations being upwards of an hour, at least. Sometimes even two. The single speed, I'd ridden for maybe a fortnight before the new bike came, but before that, it was more or less collecting dust.. So you know that I haven't been riding for too long now..
I average a speed of about 22-23kmph (~ 14mph) now, compared to a previous average of 15-16 kmph (~ 10mph). My previous recorded max speed was 31kmph and now on the new bike it's 56.5 kmph. The speed approximates are from whatever strava has told me. These approximates are on roads that are relatively flat, but don't allow too long for you to accelerate or keep your pace, I need to slow down for turns that I take so I'm of the belief that my average speed would otherwise be higher, if not for the turns. The roads also have embedded stone studs all over, the kind that ate put up on roads to allow motor vehicles to have a better grip.
Now, I ride with the saddle a little lower than it needs to be for my height (because my dad is shorter and I don't want to fiddle with it). I also can't make any structural changes or change the bike at all.
The bike is a Riverside 120. It has flat bars, a 36t single chainring and 11/34t cogs. It also has 35c tyres that are advertised as fast rolling.
Is there any way for me to be able to increase my overall average speed without changing parts? Any changes in posture? Should I try getting the saddle to my level and keeping the handlebars low to get a more natural riding position or would that be a problem because the bike doesn't have drop bars? Any other suggestions? I currently ride in the open chested posture that a hybrid is "supposed to be ridden in".