I suppose others would be better equipped to answer this, but I'll try a few things.
You ask about it being "enough." I think it's definitely "enough" to be part of an effective training plan, but I think you do a better job of getting at the right kind of question in the post when you refer to whether it is a "good way."
That "good way," as I understand it, relies a lot on context. You provide some context by showing two runs on successive days. What I don't see is how these runs fit your overall plan (particularly other workouts and weekly mileage).
You refer to the Hansons method, and I believe you've mentioned Luke Humphrey before. So you have one component of that method (depletion/fatigue across these two days, which you'll see in other plans, too) but don't include another (i.e., you go longer than I see in the Hansons method). But Humphrey is one of many who see balance as important and who caution against the long run being too high a percentage of overall volume. He's also one who warns against potential pitfalls of taking elements of multiple plans and combining them is you're not careful about why you're doing that. So those are important elements, for me, of what a "good way" is.
Is that an OK progression? It looks like it: It seems that you're building gradually. None of those goal pace runs, by itself, seems strenuous in a way that's out of line with what I've seen in a number of plans.
But it also looks as though you incorporate quality into long runs more frequently than I do. Some plans accommodate that, so if it really fits a part of your overall training, then that's probably fine. Personally, I doubt I'd have an easy time doing well in mid-week workouts and recovering well week in and week out if I were to follow a similar long run schedule. For example, I've never followed a Pfitzinger plan to the letter, but when I've followed one, say, >90% with some success, I've noted how I feel when an easier long run on Sunday is followed by an LT workout Tuesday, and there's a difference between that and an 18-miler with 8-10 at goal pace on Sunday, which is why the schedule pushes the workout to the following Thursday or Friday.
And my longest two long runs of a marathon training cycle generally don't have fast miles in them (I tend to run a regular easy long run on the faster side of what Pfitzinger recommends as a progression if weather is OK and I'm recovering well).
Are you recovering well and seeing progress? That can be tough to monitor in August heat, but you may nevertheless have a good sense of it. Are you mostly able to perform reasonably well in workouts so that you can reap the benefits and not feel as though you're racing the workout?
If your answer to that is yes, then you can probably progress to a more challenging and longer long run, with multiple possibilities for easy miles or rest on the following day.
Side note: I'm not sure about what you're saying when you refer to a 2x3. Is that some other run on a different day?
So on a more direct question you ask: I think you're coming up on 5 weeks out from your race. I'm not sure what you're planning on for overall weekly mileage or long runs for all of those. I'm assuming you're done with tune-up races, but I don't know. Can you do 20 with 10 at MP? I expect so. I just wouldn't do it more than once between now and your race, and I can't say when you would do it, because I don't know what you have planned for the other 4 (or 2-3 more substantial runs before you're really tapering). And, of course, I expect you'll be judicious about how you handle that if you're running in heat. I personally wouldn't go from 18/8 MP one weekend to 20/10 the next.
Good luck.