I really like the plank excercise and have started my wife on them as well. I do planks as part of my daily exercise when I try to do at least 2 minutes, split into 2 or 3 "sets". I've gotten up to 1:15 for the first "set". I know what you mean about the mind wandering. What I do is I close my eyes so I don't see my stopwatch and then concentrate on what my body is doing. I concentrate on my form first: where my weight is on my forearms, how high my butt is, how well I've aligned in the "plank" position, etc. Then I concentrate on my breating, deep in and out breaths. While I'm doing that try to suck in my stomach more to get better activation of my abs. Lastly, I see if I can improve anything I'm doing, i.e. "has my butt dropped too much?", "am I holding my breath?", "can I adjust my weight forward more to balance on my whole forearm instead of my elbows?". Usually, while going through all of that I reach my limit and drop.
As long as you concentrate on what you are doing, and I mean really concentrate to the exclusion of everything else, you should not get the "mind wanders". It's taken me a weeks to get close to that point and I still have issues.
If it helps, try reciting a song or a poem in your head while doing the planks. The ABCs work for me sometimes when my mind starts wandering.
Keep it up and I'm sure you'll stop having the "quit-y" thoughts.
Mini Goal 1 : 250 lbs DONE 9/23/2012 Mini Goal 2 : 240 lbs DONE 10/30/2012 - and 38" pants!!!
Mini Goal 2.5 : 235 lbs by Thanksgiving 2012 **********
Mini Goal 3 : 230 lbs
ULTIMATE GOAL: 200 lbs and/or < 36" waist