Well, I've rented a bike again and rode it for six hours (not non-stop). I have observed that
-I can't climb gradients. When I try I just grind to a halt.
-I'm a bit scared of descending from gradients. Thankfully the brakes allow me to control my speed. If they didn't work I'd be dead. I also learned not to apply the brakes abruptly or I'll still fall off my bike. (Thankfully I didn't hurt my knees)
-Sharp turns like in the picture below are a bit difficult for me.

But still, at least I learned a new skill that could improve my life. I can now get to places 3 times faster than if I walked. I'm thinking of buying a bike but I don't think it's worth it yet. I'm staying where I am until mid-September before returning to the UK. But where I am, a country bordering the Equator with 30C temperatures every day, is a cycle-friendly place, unlike London where trying to cycle is like fighting cars.

P.S. I think the "Jalan" has given away the location!