Agree with others here. Your low mileage is your main problem. Unless you have more talent - and, being honest, your PRs don't suggest as much - it's going to be hard to run sub-3 on 40 mpw.
I like the idea of marathon-pace workouts. But 6-7 miles at MP shouldn't be a struggle unless you're doing it at the end of a long run. It should feel somewhat relaxed - comfortably fast - if it's the only mileage of the day. But you already know that since you've raised it as a concern.
I don't really know the Higdon plan; does it call for tempo runs? I like tempos of 4-7 miles as Pfitz calls for - running at this pace can help make your MP feel easier. MP should not be the fastest work you're doing when you get into the meat of your training cycle.
Like others advise, I would use this cycle as strength building and then target a fall cycle for sub-3. I would buy Pfitzinger's Advanced Marathoning if only to get different ideas. And if you want to keep your mileage on the lower end, I would buy the Hansons book. That program focuses on MP workouts and repeats.
For some comparison basis, my short-distance race times were roughly the same as the ones you listed when I broke 3, but I ran a 1:24 half about 10 weeks out. I was averaging 65 mpw with several 70+ weeks and did 4-8 miles at MP at the end of long runs. I knew sub-3 was likely when I ran a half marathon at MP a couple weeks beforehand and felt relaxed throughout. Your goal should be to get comfortable at MP, which I think starts with more mileage and is helped by tempo runs. Good luck!