First off, I think you are shooting for too slow a goal given your other performances. I ran 2:30 high while in 15:30/32:00 shape and was disappointed with my performance. I get wanting to pick a goal that is achievable, but who knows how many marathons you will get in during your running career.
While the fact that you are already training at relatively high volume suggests that maybe your training has less "room to grow" when you move up in distance, I would also note that your 3k and 10k times are pretty well lined up with one another, while your 5k time is the class of the bunch. That may indicate a predisposition for the 5k, or it may just mean that you have a faster time there because it is a more frequently raced distance. So if you throw that out, the alignment of your 3k and 10k times are good evidence that you aren't all that likely to fall off considerably as you move up in distance.
Now that that is out of the way, I would say that for a late April marathon, you want to start your marathon cycle right around the new year - that would be about 18 weeks out. I have trained under a Daniels approach to training, and in that world, the first 6 weeks of your 18 weeks are spent doing some threshold work, but more heavily emphasizing vo2max work so as to improve your overall fitness before you go into the main phase of training, and so that the workouts that you will do during that main phase, which will be primarily at threshold pace, feel more manageable.
Over the 12 weeks of the main phase, one workout per week would be cruise intervals at threshold pace - starting off with mile repeats w/1:00 rest and building over the course of the cycle to intervals of 1 to 3 miles with 1:00 of rest for every mile at threshold pace. So you might start off with 5 x 1 mile and then build into workouts like 4 x 2 miles, 3 miles/3 miles/2 miles, etc. The second workout of the week would alternate and would be integrated into your long run. About 1 out of every 3 weeks it would simply be a straight long run of 20-22 miles at easy distance pace. This would come during your peak mileage weeks so that you could offset the demands of the high volume with the a decrease in intensity. About 1 out of 3 of those workouts would be runs of 13-15 miles at M pace. These tend to be the most challenging for me, so they tend to be scheduled for my lowest volume weeks. The other 1 out of 3 weeks would be the classic Daniels TLT (tempo-long-tempo) - a 20-22 mile effort run as 2 mile warm up, 4 miles of cruise intervals, 8-10 miles easy, then 3-4 miles of threshold work (either as a straight tempo effort or as cruise intervals - I tend to do it the first time as cruise intervals and then build each time into a longer effort until I get to the last one which I like to do as a 15 minute continuous tempo effort).
Not everyone loves this approach, but I have had success with it as both a runner and a coach.
I think it would be something for you to consider since there are only two quality days per week and that gives you a bit more flexibility when doing the mid-week workout since you have more time to recover than you would if you did two workouts and a dedicated long run. I also think that cruise intervals create a lot of flexibility and allow for improvisation since you can adjust very easily on any given day, yet still accomplish the physiological goal of the workout by focusing on the core idea of "8 miles at threshold pace in some combination, with 1:00 of recovery for every 5:00 of work at threshold pace. " So if you go out with the plan to run 4 x 2 miles, and you feel awful, you can adjust to 7 x 1 mile, or 5 x 1.5 miles or whatever. Similarly, if you get out there and feel great, you can shift it to 3 miles/3 miles/2 miles and get the added mental benefit of focusing through longer intervals. So there is a fair amount of flexibility to allow you to shift on the fly.