There nothing wrong with feeling proud of a near 3 hour marathon finish. But it's wrong to think, in absolute term, that it's more impressive than Greg's claimed time. To me Greg's times, assume True, are more impressive. This is not to say that a 3:07 marathon cannot be more impressive than Greg's times for someone who overcome great difficulty in achieving it. It is very unlikely, given a 16:21 5k, that the person couldn't break 3 hour for the marathon with straight forward training.
A recreational runner who’s close to breaking three hours has plenty to be proud of — it’s not something that an untrained individual could simply roll out of bed and accomplish.