The drive from OKC to Fort Sill/Lawton is an easy 75 minute drive. You'll actually pass Fort Sill to get to the Hampton Inn in Lawton, I believe it is only about a couple of exits after Fort Sill to the Cache Road exit that will get to the Hampton Inn. Hope this helps.
Edit: Note there are a couple of toll stops along the way on the 44 freeway. The freeway is easily accessible from the airport and there are signs that direct you towards Lawton.