Randy, I have turned on the computer and see this thread is long, which means some of this may be repeat stuff, but I am not patient enough to read it.
1. I don’t believe your current age is acting as a sort of governor. In fact, you will probably experience some of your biggest gains over the next couple years. I came back to running at 38 and at 42 am still finding room for improvement and learning.
2. Plain and simple, if you want to break 3:30 for example, you need to condition yourself to run a lot of miles at 8:00 pace. At least that’s the way it was first presented to me by Pski and it worked. Sure, it seemed like a stretch at first, but that’s the whole idea for some people.
3. Condition yourself to run a 20 miler about 3 out of 4 weekends. Run one of them fairly hard and the other two as they come. Spend the other weekend honing some racing skills over shorter distances. Occasionally push the envelope and do that 24-30 miler for fun. Run a 50k trail race for grins. You’ll get a new appreciation for running and begin to see how 26.2 isn’t that far.
4. Amongst doing all that, train like a 10k runner. Once you have the engine primed to where you can run a 20 miler at the drop of a hat, start thinking speed my friend. Doing faster stuff can be fun. It increases your “cruising” speed you need to hold 8 minute miles.
5. Once you can turn 60-70 a week for a month, spend 6-8 weeks at something like this and then taper two-three weeks.
Sun = 6 easy
Mon = 5 easy
Tue = 10 w/ 8 x 800m (3:30)
Wed = 13 easy
Thu = 10 w/ 4 at 7:30 pace
Fri = 4 easy
Sat = 20
In summary, train the engine and all that goes with it to cover the distance.
Then, start thinking like a fast guy and work on the turn over. The biggest advantage I have over you is that I extensively raced at shorter distances in HS and college. You would be wise to do the same and explore some local races to mix it up. I race 5k to 100 miles, so there is no limit to the fun you can have.