Hi, we're from SE Indiana and drive to WDW all the time.
Not sure where you are in the North though. (It's not shown in your profile). And can't imagine where you'd be where driving either I-65 (Gary) OR I-75 (Toledo, OH) would be an option. Unless you meant you could take either I-65 OR I-74 to I-75 and then to WDW.
In any case from where we are, right on I-74, 50 miles up from Cincinnati, it's a long haul to make it to Macon, so from wherever you are, it could be even further.
Assuming you would be going through Indianapolis, I can tell you (per my mapping software), from the intersection of I-65 and I-74, it's 1021 miles to WDW via I-74/I-75, and it's 988 miles via I-65/I-24/I-75.
In my opinion, the I-74/I-75 route is the better one, even thought about 25 miles longer. There's a good loop around Cincinnati, and Lexington. The worst stretch will be at Knoxville which you must try to avoid at rush hour at all costs. This then takes you south of Chattanooga which is where I-24 joins from the north, and is where you would be coming from if you came via I-65.
Going via I-65 from Indy takes you through Louisville (not around), and traffic there can be really bad at times. It also takes you through (or around Nashville), but even the loop there can become conjested.
You will have fewer mountains via I-65, but unless you're driving a semi, that shouldn't make much difference to you.
From Indianapolis to Macon is 613 miles.
I haven't driven the I-65 route since over a year ago. (We took that to Alabama first, but you won't be going that far West). Just got back in October using I-75. There's no real construction of any consequence going on right now. We might have had 10 miles worth of 55 mph construction zones total.
If you're going to hit Atlanta outside of rush hours, then take I-75 straight through. If you'll be there at rush hour, then take the loop around.
Stopping in Macon will cost you a few miles as the by-pass is quite a bit shorter. Depending on what time we get out, we'll either stop just south of Atlanta, or if we're doing good, we'll go to Cordelle, or Tifton.
No matter which route you take, depending on how you drive, you should be able to average 60 mph easily, driving time. From Indy, that would put Macon at 10 hours. Figure 2 gas stops at 15 minutes each, 1 food stop for 1-hour, and 2 rest area stops at 15 minutes each, you'd be looking right at 12 hours to Macon from Indy. Once you hit Tennessee, you'll have 70 mph speed limits instead of 65, so you'll be able to go a little faster then. My overall estimate is no more than 12 hours from Indy, and probably no less than 10. You can add on whatever extra it is from your location to get to the south side of Indianapolis.
From Macon to WDW is 375 miles. Traffic usually goes quite well. You've got 70mph limits in both Georgia and Florida. You can estimate about 6 hours, not including stops, and if there's no traffic problems, to get to the Disney area.
Have a great trip
