Upon sending my e-mail inquiry, the Singapore Botanical Garden (SBG) replied almost immediately "Greetings from the Singapore Botanic Gardens and thank you for writing to us. The distance between Bukit Timah Gate and Tanglin Gate is approximately 2.3 km if you follow the blue dotted route marked in the attached map.” I was impressed with the speed and sincerity of the reply. The dotted line is a meandering road inside the garden. If a straight diagonal line is drawn, I figured it was about a 2 km stretch between north and south ends of the park. The Bukit Timah (tin hill in English) Road runs into the north gate and the MRT stop and is the longest road running from north to south of the island.
The Bukit Timah pamphlet I picked up at the old Ford Motor factory, the historic surrender site of the British to Japan (my host drove me there on the 4th day), offers quite a lot of information. The 163 meter Bukit Timah Hill is the highest point in Singapore and it was where the Shonan Shrine was built. Today the area is a nature reserve along with the neighboring Bukit Batok Nature Reserve.
One of my hobbies is to visit old trees. I visited many exceptional trees - redwood, sequoia and Moreton Bay Fig in California and others in Australia, New Zealand, Japan and Taiwan. The Bukit Timah pamphlet read: "The number of plant species growing in the Bukit Timah Nature Reserve is more than in the whole of North America and at the same time, Singapore's oldest tree, a 400 year-old Seriya Shorea curtisii Tree is also found here. This tree is possibly the Temak tree that gave Bukit Timah its name."