Train for trail marathon after trail 1/2

I just ran my first trail half marathon (and actually, my first 13.1 period). I'm eyeing a 20-ish mile trail run in August (about 3000 gain), and then, hopefully, a full trail marathon.

So I have some time to ramp up, and I'd like to do it intelligently. I currently run 4 times per week, with a long trail run on the weekend, and 3 shorter road runs during the week. My pace on the trail is usually 11-13 minute miles, and I ran the 13.1 race at about 11:50 pace.

How should I start ramping up? Do I simply find a road marathon training plan and just make my long runs on the trail? There don't seem to be many trail-specific marathon plans out there.

Do I kind of just wing it, and go with something generic like "3 weeks of steadily increasing, then 1 week layoff" until August?

Long story short: any input on training a trail 1/2 marathoner for a trail marathon?

Thanks!