1

It is possible in one very long day-sleep on the bus!
There are buses leaving for Çanakkale almost every hour with several bus companies including Truva Turizm,Kamil Koç and Metro Turizm and it take approx 6 hours.So if you catch an early morning bus you can be there for 09.00 or 10.00,spend the day looking around or if the budget allows book onto a short tour for the main sites and then catch an early evening bus back around 19.00-20.00 getting you back into Istanbul again early hours.Catching an unsociable night bus down would mean you can maximise your daylight hours around the peninsula.
http://www.ebiletal.com/truva/sefersorgula1.asp
https://secure.kamilkoc.com.tr/SeferSorgula.aspx?k=1094&v=1133&g=27-6-2012&yks=1
https://online.metroturizm.com.tr/buyticket.aspx
As you can see there are lots to choose from!They depart from Esenler Otogar on the European side of the city.

2

Wow- that sounds slightly hectic!

Has anyone been with one of the tour companies that offer the 1 day tours from Istanbul?

thanks!

3

This Eceabat based outfit has a good rep for Gallipoli daytours and does one out of Istanbul
http://www.crowdedhousegallipoli.com/gallipolitours/gallipoli_tours.html

4

It isn't really that hectic but by catching a very early bus you give yourself more day light time to see the area.Tours from Istanbul set off later,around 06.00 and get back around 23.00.Travelling time is the same but it just means you have less time once there to look around.Most agencies such as Crowded House,Anzac House,TJ's etc simply book you on to these regular buses and meet you at the bus station once you arrive and put you back on it to go home again.
By all means contact one of them to book a tour for when you arrive if you wish but by going down on the bus you choose you an give yourself more time.Time you can use to look around Çanakkale before the tour starts.