[Firebird]

There is one thing you should make sure before heading out there and that is that you are in season! When i was there the lavender season was over, though the Autum field was also beatifull.
The "famous" farm there is the farm Tomita wich has its own train station from where it is a 10 minutes walk. (its around 20min by scenic train from Furano) You will have to take the Shinkansen from Tokyo to Hachinohe and than change to the Limited express to Sapporo, from there youll probably have to go by local train. hyperdia says around 13 hours from Tokyo to Furano. I would suggest you head out in the morning and go up to Furano. Stay the night in Furano and head to the Farm in the next morning. If you really have to get back on that day i can only suggest a sleeper train from sapporo to Tokyo, wich will cost you extra if you travel with a railpass, otherwise it will be around the same as the shinkansen/limited express going up.
I would suggest that you dont head back in one day, but in 2. Choose a place between Furano and Tokyo wich you would like to see and make a stopover there ( eg. Sapporo, Hakodate,....) and than head back to Tokyo the next morning, because once you have made it to Hachinohe its only around 3 hours to Tokyo.

Hope i could help you somehow. if you have more questions feel free to ask