Q. Is it suitable for long drive - Discover 125?

This is a well engineered bike and although it fits the commuter requirements better, it can be used to cover longer distances too, at a stretch.However that being said, you must also remember that due to its smaller displacement, it is having to work harder than a bike of bigger cubic capacity. Hence it make become hotter and need to be kept under 70km speed.

Not suitable for long rides very bad performance and very bad mileage

Yes surely but try to keep the speed range 80-85. Dont try to take it to its max. Speed becoz risk of engine seize increased.