I want to thank everyone for their replies - a lot of good ideas. I think it's fair to say there is more than one way to achieve a successful result.
Anyway, my plan right now is three workouts every 10 days and running 5 out of 6 days. So far, I've done ten workouts.
I am viewing things as though there are five different types of runs: Intervals, Threshold, Marathon pace, Intermediate-Long and Long.
I am emphasizing three of these run types at any given point in time, while at the same time doubling down on a particular run type as I move through the weeks leading up to the race.
So, what I've done so far has been Intervals, MP, Intervals, Long. (have completed this cycle 2 1/2 times far)
As a starting point, I went with VDOT 43 from Daniels (VDOT 45 equates to a sub-3:30 marathon). So, to me that VDOT 45 is an eventual target for my runs. When I meet my workout target, the next time I do that same type of workout I either increase the VDOT by one unit or add one mile to the workout (but not both at the same time).
On Intervals, I'm always running 6 x 800 with a 1/4 jog in-between. Had trouble with the very first workout and dropped back to VDOT 42 for the next one but then hit three interval workouts in a row so that my last one was VDOT 45 (target 3:24 but I was only able to run 3:27). That said, it seems my in-between jogs are slightly less than Daniels seems to suggest so I'm thinking of moving it from .25 mi to .30 mi. Provided I hit this next workout on Friday, I'll then try to back it up with a 5 x 1000 workout about a week later. If successful, I'm thinking of then dropping Intervals from the program and replacing those with Threshold runs. At this point, two of every four workouts will be Threshold, with one MP and one Long.
Eventually, I will change that to one Threshold, one MP, one Intermediate-Long and one Long.
My last MP run was five miles (7:52, 7:49, 7:43, 7:47, 7:41) - my race target is 8:00. As I keep increasing these MP runs one mile at a time, should I be trying to run these as fast as I reasonably can or should I hold myself more to running around an 8:00 pace? Is there any thought to trying to get used to more of a 7:45 pace so that 8:00 will feel that much easier on race day or is it better to be running 8:00 all the time to get my body used to that?
I am expecting the MP run distance to eventually peak at 15 miles (that's a big difference from the Hansons program that I ran last year, which was 10 miles) so that seems like a lot to handle.
Long runs I am increasing one mile each time (since my easy runs were 8 at the time, my first "long" run was 9 and I've since done a 10 as well. I'm doing these as progressive runs and trying to do the last third at MP. I am not sure where to eventually top out - the longest runs from Hansons were 16 and Daniels says don't run for longer than 2 1/2 hours (which might be 17 miles for me). But, I'm getting feeling I need to be doing up to 20 (maybe more but at some point I feel I might be pushing my luck). If so, how should a 20-miler be run?
How should Pfitzinger's Intermediate-Long run be done? (I think I'm going to get his book but haven't done so yet)
My easy runs were only 6 miles in Hansons. I'm at about 8.5 now but am planning on eventually getting these up to 10-11. Daniels easy run pace at VDOT 45 is 9:17 - most of my easy runs are a little bit faster than that. I'm feeling my easy runs are about 15 seconds faster (with the same effort) than they were last year.
If I'm going to go under 3:30, I think this is the year. At age 55, theoretically it gets harder every year but my training has been far enough from optimal that I can overcome the otherwise year-by-year decline.