Hola Ashlyn

You can't say "dentro de" here because "Llegué en solo dos horas" means "it took me just 2 hours to get there" - with dentro de we need a future event and this is in the past "llegué".

You can say for example: Llegaré dentro de dos horas. (I will get there in two hours) - this means that if it is 5 o'clock you will get there at 7 o'clock. 

Saludos