I don't think you can go that way as I'm pretty sure there's no connection between the turnpike and 439.
You can get into Disney through the back door though. From the Turnpike, exit and take hwy 50 east toward Winter Garden. You'll go about 2 miles and then hit the junction with 535. Turn right (South) and follow it right to Disney.
Once you're on 535, it will be 13 miles to Disney. A word of caution. This is not a main entrance and there aren't any signs, so you could drive right past it if you're not careful.
Use your odometer carefully. Right at about the 13 miles, the road is going due South and then will make a 90 degree turn to the left (East). At that point if you go straight ahead it will put you on Dopey drive going right into Disney. Go to Vista Blvd, turn right (left will take you back to 535). Once on Vista you can go straight to go toward the Ft. Wilderness entrance and the security booth, and eventually straight to the MK resort area.
If you're going elsewhere, you can turn left off of Vista at Bonnet Creek and go up past the Port Orleans resorts, Old Key West, and eventually connect with Buena Vista Drive.
An alternative to this final segment is to not follow Dopey Drive, but make the left turn on 535 and go another 1/3 mile east and then turn right onto N. Buena Vista Drive. You can follow this all the way past Shades of Green, the Institute, and to Downtown Disney.
I presume that once you're on Disney property, you'll recognize your way around.
I know this route because we just drove it last May for the heck of it. We didn't drive it going in, but rather going out as I just wanted to see what it would be like for future trips. It's mostly 2-lane road, but misses most built up areas. You'll be able to drive about 50mph or so.
FWIW, there's another way to get to Disney that avoids I-4. We've done this dozens of times.
From the Turnpike, take Hwy 27 south, toward Clermont. Continue south to junction with 192, and from there go East to the Disney exits. Hwy 192 construction is now all completed and it's all 4-lane roads. The traffic is not bad at all anymore.
The back road will get you there, but if you have any concern that you'll get lost, then the hwy 27/hwy 192 route is a better one. Just be careful on hwy 27 as there are some traffic lights that can come up on you pretty quick.
Hope all this helps. Have a great trip. We're going down again ourselves for January.