Sorry for the long rambling post. I think I make a point or two in here somewhere.

If you're position is anything like mine Scott you'll probably need to tackle it in two parts.

About 8 months ago I moved off of a second shift IT job that was very much a "cranking widgets" sort of gig (to use the GTD term). My new job is 7-4, and a whole lot more complicated.

When I was on the old job I had a bunch of side projects that took up spare time (and I may or may not have occasionally worked on them while I waited on things to process at the "day job"). For that old job I was able to run it almost entirely on auto-pilot brain mode, and easily kept most of the side project work in my head. The new job is more complicated by orders of magnitude (multiple projects, multiples layers within each of those projects, etc), and I've still got all those old side projects running in my "spare" time (not to mention all the family things that a wife and three kids brings to the table).

After about 3-5 months at the new job I realized things were getting more and more complicated and out of my control and that I wasn't doing a very good job of managing them. However it has only been in the last month or two that I've been looking at GTD as a way to organize those things and get them into a system so I don't have to keep it all in my very tiny brain.

Soo.... getting back to your question and the point. I've got the issues of:
A) I have things I need to get done that have to be done during the normal business day. How do I handle that?
B) I have so many more things period that need to get done. How do I handle THAT?

I certainly don't have a pat solution here. But what I'm working towards, and seems to be working thus far, is setting up two GTD sub-systems: A work one for all those projects and items on the mind, etc; A home one that handles all those side projects AND all the bills and home repair items that need to get tracked as well so when I'm home and not working on them I can just focus on the munchkins and enjoy them. To sync the two up I'm tracking them all digitally (that method is being tweaked and worked on... currently on my iBook using VoodooPad Pro, though not sure if it's going to be the final solution to the problem).

I'm about 75% there with the work system. The home system is only at about 25% as all that I've got working there is the digital part and nothing for the paper bills and magazines and such. There is something to be said for having all the projects/tasks out of your head and into a system of some kind. I'm not organized by nature, and I'm not organized enough yet to say "I'm so much more productive thanks to X or Y", but I am making progress by getting the day job stuff organized so I can at least leave it at the office.

Part of the solution for you may be (and if you?ve read this much of my ramblings you?re probably not TOO ADHD) to schedule yourself for only ?-3/4 of your day and allow that time. One of the suggestions I?ve seen elsewhere on productivity is the 48/12 rule where you work diligently for 48 minutes on one focused task (or maybe a couple smaller focused tasks) and then take 12 minutes to get up, go get a cup of coffee, do some exercise, or just change scenery. So applying that to your problem may look like 48 minutes of day job and 12 minutes of taking a checkbook and stack of bills to the break-room, or after you get up and walk around for 2 minutes sit back down and take 8 minutes to call the mechanic about your brakes before walking around for another 2 minutes before getting back to the next task.

Somebody else mentioned this as well, take your lunch break and plan half of it to eat (because you?ve got to eat man!) and the rest to take care of those personal items that can?t wait. To some degree it may mean changing the way you time your workday and planning in an extra half an hour to handle those things in the middle of the day ?off the clock?.

The other part is probably going to be in improving your estimation of completion times. The rules of thumb I?ve heard is anywhere from double to triple the amount of time you expect to take for a project and use that for scheduling. This is going to apply to work projects and personal ones. If you think you?ve got 5 hours of work planned for the work day, then don?t try and fill in the other 3 with more work because you?re just setting yourself up to fail when the 5 hour project takes 10 instead.