Sepang is about 20mins away from Seremban westward. Very close to the new KL Airport. To get there by car should be about 3-4 hours depending on how fast you dare to drive without being caught. If you are new to driving in Malaysia roads and highway..I would advice caution and drive with care.
But also note there are road road along the way .... as usual they take ages to repair or widen the road. They have been doing quite a bit along the way at Malaca, Port Dickson..etc Have to check with anyone who have driven up towards KL. They should be able to tell you the latest on the highway.
If you maintain a constant 120 to 130kmh, then it will probably take you 3 hours plus to reach sepang. Depending if you are driving up alone of with a few cars, best is to get walkies to facilitate communications.
Best is to start your journey early...traffic not so horrible and weather is not so hot...can stop by Exit 209 for Breakfast too.