Overreaching on Long Run

Normally before a marathon 3 weeks before or near to, its important to reduce mileage, this can be the reason the watch is telling you this messages, do you have a training plan for the marathon?

5 weeks out so not time to taper yet.  Not following a plan exactly, using GSW as a guide.  Next week is 4 weeks out, was planning 20+, then start the taper.

Overreaching sounds bad but isn't per se, in fact it's an effective training concept. Non functional overreaching and overtraining is bad and you will get there if you don't recover properly from overreaching sessions.

Overreaching, is just the training effect note based on heart rate profile during run and duration of time at % of max (Usually coincides with Aerobic Training Effect of 5.0 of 5).  Could just be a flakey optical heart rate or a sign you were pushing it very hard during the run.  You are in peak strong training... so a bit of strong and long efforts aren't so bad.  However most people push too hard on long runs, looking for most/many of them to be in the 70~83% of max maybe, if really warm it will cause HR to be higher for cooling or towards the end of the run, so often HR for the same pace might end up being 5-10beats higher in mile 18 vs mile 6.  
Using conversation as a guide can be helpful also ,if you can some what easily say short sentences or sing to your self... then often that is in your aerobic range as well.  

I would also assume (not sure) that even if you were in a nice aerobic long run effort and HR... that if your recent Acute Training Load is at a certain value... but then you go for a monster 3hr run that is more than half your weekly TrainingLoad... it would still say "Overreaching" instead of "Base" or "tempo"...   For instance my 10mile run this weekend was just an aerobic run, 74% of max HR for me... 78minutes gave me a Load of 151... for me that is a ok Load considering my 700-1000 TSS weeks over the last many months.  Kept my weekly load within the 'optimal range' and 151 wasn't super high for me.  However for other people with a typical Chronic load of only 300TSS, that 151 might have been considered big/Overreaching even if it was at a zn2-zn3 effort.

Does your HR of that activity and % of your max seem to correlate with your effort on the run?  breathing super hard or easy?   gasping?  Does the HR seem to match effort within the run as well... go down when going down a hill... up when pushing it up a hill... or does it just float around willy nilly (sign of bad HR sensing).

SteveI said:

but now I'm not sure what to do.

Recover well from your run(s) before the next run, following the Recovery metric.

The watch as 3 types of guidance to optimize your training load: training load, acute load ratio and HRV status. However, if you have been going too hard for too long already, you may have pushed up all the safety zones and you may be overtraining.

Keep your training load, the acute load ratio and the HRV status in the green.

In short, don't overtrain. It would be too bad to get injured that late in your training plan.

I understand you have your own training approach. It also seems you are not following a race plan from the watch. Remember, you don't have to train everyday even if the watch will suggest a workout.

If I were you, I would look up the last few weeks of a marathon training plan adapted to your capabilities and age, and I would follow that plan.

One other thing to note after looking into it just a bit with my training and some garmin pop-up tips.  Couldn't edit my old post, errors...

 Bit more clarification to the message of "Overreaching" for an activities Training Effect.  If you select the '?' for more info on the website it gives some more info from that area of Stats.  This will be in either the Aerobic or Annaerobic TE , so for a long run with some Tempo... might be "4.7 Highly Impacting"  .... but then a super Hard long run with Tempo miles in the heat, or a Marathon Race... will likely be "5.0 Overreaching" --- looking back at my training that was the case, a few long runs w/work... 4.5-4.7... race was 5.0.  I had some hard Threshold interval workouts (3x2mi or 2x3mi ... that hit 5.0 iirc).  Usually 5.0 is associated then with "Recovery TIme... 52hrs" or something high.

It does not have to do with the Training Status feedback seen on main page.  "Maintaining / Productive / Strained / Peaking" etc.  Which looks at a lot of training metrics like acute/chronic load, recovery time, HRV , etc.

(GARMIN's help about the Training Effect score levels below)

Planning How You Train

When performed with a compatible Garmin device, each activity will generate a Training Effect score, along with a message that helps you understand how the activity impacted your fitness.

In general, a higher Training Effect indicates a greater impact on your fitness, to a point. A Training Effect of 5.0 can actually be harmful if reached with regularity and without adequate rest and recovery time afterward.

Thank you everyone!!  I read all the comments, and you guys are spot on.  I looked at my history, and most of my long runs were in the 4.2-5.0 range.  I am pushing distance 5-10% every other week.  Also, in the last 2-3 miles, I pick up the pace to marathon pace, to try to "learn how the race will feel.".  Since I am upping the miles higher than I had run before, the extra strain is the cause.  I do feel like I am improving.  While I am following a mainly zone 2 principle, at the core, the teenager (athlete) in me, still has the no pain no gain mentality.  

Also, my HRV is low, in the orange. It always seems to be in the lower end.  Does this mean I need more rest?

Acute load is optimal.  Typical weekly mileage is 40-50 miles, and I only do 2 sessions uptempo or sprints.

Training status is Maintaining, likely due to HRV.  But even when HRV is green, status was maintaining.  Status has not been productive for nearly a month.

I've looked over many plans and listened to many many experts (some questionable).  I don't fit any guide well.  I can best describe myself as a beginner pushing hard / quasi intermediate.  I am 56 male, and just got back into running in the past year.  Last race was @ 18, 10k in sub 36. That was 1986. I went from running a 5k in november in 33 min, to running 25 min in a threshold workout last month (GSW said 10min warm up, then 30min @8:20, then 10 min cooldown). I just felt very good, and pushed the pace slightly harder.

Next week will be my last long run, and I go by time.  My last long run was 3hr 30min(20.3miles).  Next Sunday will be 3hr 40min(est. 21miles).  Based on everyone's guidance, I will do it, with rest before and after, and I will stick with the easy pace, and not push at the tail end.  I ran a half in the spring, hills and heat left me dissapointed.  I have another half in 2 weeks, which I will run at marathon pace (sub 2hr).  If I feel good at the end, I know I am ready for the full distance, shooting for sub 4 hours.  If the half is brutal, I know to lower expectations.