As soon as the Istanbul Metro starts running (about 06:00 am), go out to the Büyük Istanbul Otogar (bus terminal) and find a bus to Gelibolu (for the Gallipoli battlefields) or Çanakkale (for Troy). More...
For Gallipoli, when you arrive in Gelibolu, consult the bus schedules for return trips to Istanbul so you'll know when you must get back to the bus station for the return journey. Then, negotiate with a taxi driver for a tour of the battlefields, specifying exactly the places you want to visit, how long you want to stay there, and for your return to Gelibolu by a specific time.
Then negotiate with a taxi driver for the 30 km/19 mile trip to Troy archeological site, an hour or two of waiting time, and the return trip to the bus terminal. (Yes, this is a bit expensive, but if you want to do this excursion in a day, you'll have to bear the cost.)
The entire excursion may take 14 to 16 hours, with about three to four hours at the Gallipoli battlefields, or two hours at Troy.
—by Tom Brosnahan