Hiking or trekking unplanned is not a very good idea.
Learned this the hard way. Went to an impromptu trek at the McRitchie Reservoir without proper trail shoes, socks, and just a bottle of water from the vending machine. My goal was the Tree Top Walk which was a good 5km away from the starting point.
I didn't really thought that I could reach the Tree Top Walk in time (it closes at 5:00pm), but I'm just glad I did.