A 2 hour half marathon is 9:09 per mile, or 5:41 per kilometer, for 13.1 miles. At that pace you pass 5K in 28:26, 10K in 56:53 and 10 miles in 1:31:32, with about 5K left to run in 28:28.
Here the rounding is on your side. The exact pace is 9:09.2 per mile, so the rounded 9:09 held for 13.1 miles finishes in 1:59:57, 3 seconds under two hours. One second slower, 9:10 per mile, finishes in 2:00:10. Kilometers work the same way: 5:41 per km finishes in 1:59:54 and 5:42 per km in 2:00:15.
That is not much of a cushion. A crowded first mile or a watch that reads long can use it up: courses are measured along the shortest route a runner could take, and at 9:09 per mile each extra tenth of a mile you cover costs about 55 seconds. 9:05 per mile finishes in 1:59:05, 55 seconds to spare.
A half marathon is exactly half a marathon, so 2:00:00 is the same pace as a 4 hour marathon. Holding it for twice as long is another matter: Riegel's formula, which the marathon time calculator uses, puts a 2:00:00 half level with a 4:10:12 marathon, and with a 54:23 10K and a 26:05 5K.
The tables give the clock time at each marker for an even pace and for a 2% negative split, which runs 9:15 per mile to halfway (6.55 miles, reached in 1:00:36) and 9:04 per mile after. Its 10-mile check is 1:31:50, against 1:31:32 on the even plan. To try a different split, use the split calculator.