A 3:30 marathon is 8:01 per mile, or 4:59 per kilometer, held for 26.2 miles: halfway in 1:45:00, 20 miles in 2:40:11 and 30K in 2:29:18.
For this goal the rounding works against you. The exact pace is 8:00.6 per mile, which a chart rounds up to 8:01, and holding 8:01 all the way finishes in 3:30:11, just over. To break 3:30 you need 8:00 per mile (3:29:45) or faster; 7:56 per mile gives 3:28:00. Kilometers have the same trap: 4:59 per km finishes in 3:30:16, and 4:58 per km in 3:29:34.
The tables give the clock time at each marker for an even pace and for a 2% negative split. The negative-split plan starts at 8:05 per mile, passes halfway in 1:46:04, and then needs 7:56 per mile for the second half: the same pace that would bring an even runner home in 3:28:00, now asked of tired legs. If that sounds like too much, the even plan at 8:00 per mile is simpler to follow.
Before you commit to the goal, compare it with a recent race. Riegel's formula, which the race predictor and the marathon time calculator use, puts a 3:30:00 marathon level with a 1:40:43 half marathon and a 45:39 10K. It is a prediction for a runner trained for the marathon distance, not a guarantee.
Check yourself against the course markers rather than a GPS watch, which usually reads a little long because courses are measured along the shortest possible route. At 8:01 per mile, every extra tenth of a mile on your watch is about 0:48 of running.