Hello Katie and MM
I suspect I'm that veteran Disney runner, so thanks.
The start of the Half Marathon is almost precisely at the 1 mile mark of the Full Marathon. Warning here, last year the starting corral for the Half Marathon was one, that's correct, one lane wide, with steel traffic control barriers like you see for parades on both sides. With 12,000+ starters for the Half Marathon the field stretched back quite a long distance, and it took the folks at the end a long time to pass the start. Other posters here can tell you how long it took them to get to the starting line.
The only resorts which you can physically walk from and get to the Half Marathon start are Old Key West, POFQ and POR. This last January my wife and I slept the night before at POFQ, and we left the lobby of the resort at about 5:15. Walking at a comfortable pace it took us 20-25 minutes to get to the starting corrals. The best part about this walk was Disney had lots of port-a-potties out for the next day's Marathon, and there were no folks and no lines for them, so we hit them just before we got to the starting corrals.
There were perhaps 20 people in total in our little group who were walking to the start of the half, from all 3 resorts. As we got closer to the Half start we saw the masses coming from the left, from the holding area in the EPCOT lots. They literally seem to have been released the same time we arrived, so we had no trouble getting into our corrals.
Good luck on January 6.
Craig