The estimated perimeter of Pulau Ubin is approximately 23km.
At an estimated rate of 1 hour per 5km, this endeavour would take at least 4.6 hour. Since I am very unfit, perhaps this will be closer to 5 hours or even 6 hours, while also having to find one's way around as there is no actual footpath or route.
Next, I will be looking for cheap GPS real-time receiver and an old map of Singapore. A GPS receiver will be much more accurate than my iPhone and can be used to map out the actual route taken around Pulau Ubin. I would also be able to go geocaching or make up some geocache variant. Looks like prices are below SG$100 so its not a costly investment either.
The Global Positioning System uses a set of 27 satellites (24 are in use, 3 are backups) which orbits earth at about 19300km above ground. At any one time and from any location on earth, there will be at least 4 satellites within "visible" line-of-sight. The GPS receiver sends a signal trying to locate at least 3 of these satellites and from the signal it receives back from the satellites, it uses trilateration in 3d space to calculate the actual position of the GPS device on earth.