Training plan for first marathon as an experienced runner

I've been running for 15 years total, with various levels of commitment. I've run a bunch of half marathons, all were under 2 hours, but I'm not fast and I haven't run one officially in a couple of years. PR of 1:55, I'm confident I could do one faster now but I don't have proof.

I'm now considering finally running a marathon. Where I live, a lot of them are in December/January. The one I'm most interested in (Mazatlan) is December 6th, so I have 3 weeksish to decide whether or not to do it if I want an 18 week training block, a little long for a 16 week.

I currently run 60km a week(ish) barring a monthly deload week, with 5-6 days of running + at least one strength day + power yoga. Although this would be my first marathon, I'm definitely not a beginner. I'd also like to keep doing speed work, mostly because it's my favourite kind of workout! I don't really need to run hills as there aren't any nearby and it's a flat race along a seafront. I like running hills, I just don't like the track it takes to go find them.

The other confounding factor is that I live and train at altitude, and the race will be at sea level. I know this is a huge plus for shorter distances, but I'm worried it'll make me overconfident for a marathon and I'll go out too fast to sustain. Either way, it makes choosing a goal pace

I've been focusing on 5/10km training for the last few months, so my longest recent runs were 11 miles/18ish kms. (Sorry about mixing miles and Kms in this post, I'm British and honestly we're nasty like that.)

So I guess, what would y'all do if you were me?

I'm considering hal higdon novice 2 + adding some speedwork, but then I've basically built myself a DIY advanced 1 so maybe I should do that? Or should I get on a pfitzinger plan? I definitely don't want to do the Hanson method as topping out at 16 miles for a marathon sounds terrifying.

Am I being an idiot and should I choose a less advanced plan? I think I'm an experienced enough runner that if I start a plan and realise it's a reach, I'll be able to bump myself down sensibly to a less challenging plan.

Fuck it, maybe I'll just do the half and try for a massive PR?