Considering how long you've been out of school I don't think your grades will be weighed very heavily. I think the GMAT is much more indicative of a person's IQ than their GPA, adcoms know this. You have a solid GMAT score, and if you're above your schools average I wouldn't take it again unless (a) you know you're going to score better (b) it's not going to take away time that would be better spent working on the rest of your applications, and it's not going to take away from time that would be better spent with your family.
I used to think this way too, but I've since changed my position. The problem actually is a very simple one. You may have a 680 and a 2.5, but someone else will have a 680 and a 3.5. You may have 10 years experience, but so will someone else.
That being said, part time programs ARE different - your GMAT matters less then as it is not reported as part of the official average. However, your poor experience as an undergraduate will be of concern to the adcoms, and while your GMAT score may mitigate this, you will need to adddress it. Simply saying it was 10 years ago is not enough, because right next to you is someone else who got a 3.5 10 years ago and thus represents less "risk".