Choc. Stout extended mash

Well I have quite a busy day, so I thought I would try my hand at an extended mash. Picked a great recipe too... A clone of Young's double chocolate stout. I'm worried about overattenuation and getting a dry version of a stout...
By the recipe, it should be mashed at 153F for 60mins. I got mine to 156F and decided thats where I'll leave it. I have no idea what time I will be home later, but I'm hoping for less than 6hr mashing, maybe only 4hrs.

Quite an experiment I have on my hands....
...stupid fantasy football draft... grumble grumble... labor day... being on call for work.... grumble grumble...

Wish me luck, and lend me your insights! What do you think I can expect, and what will I be able to do when I get the boil going?
One thing I was thinking... to compensate for the possible dryness, would be to increase the lactose. I have 12oz planned, but could increase to 16oz if neccesary. I do have dextrose laying around too... maybe 4oz.
What do y'all think?

The 156F will denature a lot of the enzymes, so I think you'll be fine even with the extended mash. I would hold off on adding any extra unfermentables until fermentation is complete. You can always add them later if it comes out too dry, but there's no way to remove them once they're in there. Just my $0.02.

Excellent advice, thank you!