I think you are refering to Lembah Kiara at Taman Tun. Indeed it's a nice place to jog. It even has soft running tracks. With greens, lake, man-made waterfall and fountain there, there isn't anything else to ask from it.

Enter Taman Tun from the Balai Bomba area and go straight till you reach a traffic light. VADs (previously IBM) is on your right. Go straight a little further. Trelessis apartment is on your left. At the end of it, turn left and go straight a little. You will reach a school on your left. Lembah Kiara is just opposite it on your right. Beware of monkeys though.

A lot of runners train in the morning starting from Bukit Aman carpark at Lake Gardens to Sri Hartamas on Saturday and Sunday morning. I am one of them who train on Saturday mornings.

Incorrect. Bukit Aman to Sri Hartamas (Petronas station) is only around 9.7KM. To get total of 21KM through and from, you need to do 2 loops around Petronas which is around 0.9KM per loop.