There are several ways to travel between Istanbul and Ankara. The easiest and fastest way is to fly from Istanbul airport to Ankara airport, which will take you 2.5 hours. The HST train from Halkali takes 5h30. There are also different buses from Istanbul that take more than 6 hours, it is not the fastest solution. By car, it will take you 4.5 hours, but it is not the most economical solution.