Edirne is about 230 kilometres northwest of Istanbul, roughly 2.5 hours by car or intercity bus. Buses run frequently from Istanbul's main bus terminals (including Esenler), and the journey is comfortable and inexpensive — it is one of the easier day trips from Istanbul, though an overnight stay allows you to do the city properly. The highway passes through flat Thrace and crosses the Meriç river as you approach.
Train connections exist but are slower and less frequent than the buses; check current schedules as service has varied. Edirne sits at the corner of Turkey where it meets both Greece and Bulgaria, making it a natural stop on overland journeys between Istanbul and Sofia or Thessaloniki. The Kapıkule border crossing into Bulgaria is just a few kilometres from the city centre, and the Pazarkule crossing into Greece is similarly close. If you're travelling by bus between Istanbul and Western Europe, the bus will likely pass through or near Edirne in any case.