If it were me, I would make several changes to your plan. I would do some faster-than-mp work in that mid-week run, probably threshold pace. I would also make the long run all easy one week and 6-10 miles at mp the next (i.e. I do 16mi easy this weekend and next weekend I do 16 with 6 at mp). Finally, I would make those brick runs seperate easy run workouts, but that discussion is maybe for a different thread. Good luck.

Rest day? Maybe take the Tuesday or Friday run out.

Tues: 6miles
Wednes: 10miles+
Thurs: 3miles?
Fri: 6 miles
Sat: 3 miles?
Sun: 16-20

Picking out mileage based on a conservative running speeds (assuming you're faster if you're already running this much), that's about 44-48 miles per week whilst also doing triathlon training. Not sure how you expect to keep up this plan for 3 months with no rest day. Up to you, but looks aggressive to me

What is your rough goal time for the marathon? And what is your background (first marathon? PR attempt? Completing vs competing? etc.)

That running plan looks harder than I can handle. If the miles don't impede your other training and you're not time constrained, then it's probably okay. I'd guess that's more mileage than most on this board run.

If you are saying "1 hour less running means 1 hour more biking" then I'd probably swap for more cycling miles And then you might benefit from a rest day in addition to that.

The basic outline looks fine. My recommendation in terms of progressing that basic week would be this:
You've done a few 15-16 milers at 8:00 pace. Over the next few weeks, add some distance to these runs while maintaining the same pace:
(i.e. Week 1: 17M@8:00, Week 2: 19M@8:00, Week 3: 21M@8:00).
Then, start to add your marathon pace segment on the end:
(i.e. Week 4: 20M w/ last 5M@7:00, Week 5: 20M w/ last 8M@7:00, Week 6: 21M w/ last 10M@7:00).

With regard to the Wednesday run, I would suggest doing something slightly faster than goal marathon pace that gradually grows in volume
(i.e. Week 1: 3x2M@6:30 w/ 1M@8:00 btwn, Week 2: 2x3M@6:40 w/ 1M@8:00 btwn, Week 3: 3x3M@6:45 w/ 1M@8:00 btwn, Week 4: 2x4M@6:45 w/ 2M@7:45 btwn, Week 5: 4x2.5M@6:40 w/ 0.5M@7:30 btwn, Week 6: 3x4M@6:50 w/ 1M@7:30 btwn)

Sounds like you know what works for you but since you asked...
- no speed work? I'd make your Tues run Yasso 800s
- no rest day? again, maybe you don't need one but I find that a true rest day really helps ensure my body has time to restore
- as you get up above 16miles on Sundays, you'll want to alter your Saturday rides... it's one thing to run 12 miles on tired legs, 18 miles is starting to ask for injury
- your Monday rides should start with ridiculously easy intervals and work up but remain in the easier side of things throughout

You'll also need to start thinking about your training post-marathon and pre-70.3. You know this, but you'll need a solid 2-3 weeks to recover from the 26.2 and you should let that happen so your body is in good shape for peak 70.3 training.

Background: have done a dozen+ marathons and 50k over a bunch of years and got into 70.3s two years ago. have since made the decision to only do fall marathons (70.3s are easier to recover from) (which kinda sucks because there are some really nice springtime 26.2s).