How do you deal with performing much worse than you hoped to in a marathon?

I never had a four month training cycle. I was always training for a marathon. But in your place I'd probably find another marathon really soon, before the effects of the four month training cycle wears off.

When I got to my late 30s I wanted to get a marathon in a non-embarrassing time on the board (I'd never raced beyond a half, and consider(ed) 3000m my best distance.

My first one I went from passing people at 20 miles to gradually bonking, and finishing minutes outside my target.

My way of dealing with it was to decide to have another go in the same race the next year.

Second time around, I had even splits of within 30 seconds for each half and was about 2 1/2 minutes inside my target.

At that point, though, I had more than 20 years of running behind me, although my "long" run prior to marathon training was generally a hard 10.

Two-and-a-half years is a very short time to build the kind of background for a marathon. I would work on getting your shorter distance times down - there appears to be plenty of potential for improvement there - while getting in the occasional long run. Faster shorter distance times will make marathon pace easier, and two or three years of building some aerobic background with something like a 20 miler once a month or so is going to give you a lot more strength.

Incidentally, my marathons were nearly 30 years ago. In those days, I'd do the 20 mile/2 hour run without a drink, and gels were unknown. Shoes were ordinary light-weight road-racing shoes.

Let me get this right.....

You are 25 and have been only been running for 2.5 years?

For the love of everything good in the world, please give yourself a break.

I ran my first marathon at age 18, but was not able to run my PB until age 37. And was even able to a marathon overall at age 40. You have lots of time to improve.

Be patient and give yourself some grace.

Lastly, doing a decent 10k or even 1/2 marathon 4-6 weeks out from your next target marathon may be a good confidence booster.

I'm assuming by "poor results," you mean "a slower finish time than I wanted."

I don't think anyone would be happy to accept that possibility, so... many people define the result of a training cycle as "the improvement you did make" instead of focusing on "the improvement you didn't make."

You ran a 3:50:xx. But before the 4 month training cycle, you might not be breaking 4:00:00 (no matter what your calculator predicted). So you certainly did improve during that four months, just not as much as you wanted to. But I have a feeling you were looking at the predicted 3:20:ish and assumed (because that's what all the training plans want you to assume...) it would only take you four months (1 cycle) to train into 3:20:ish shape. Instead, you ran 3:50:ish and feel bad about it. I get it. Heck, I've DONE it. But racing a marathon up to your potential takes more than four months (a single training cycle). So don't feel so bad. Keep at it. Or don't. In the end, it doesn't matter. Be happy doing something you enjoy.

I had my 1st marathon a year and a half before in almost the exact same time, but the course was tougher the 2nd time. Even still, it was massively disappointing. I actually remember wrong, I took 2 gels, not 1, but even still it wasn't nearly enough. Honestly, a big part of the ego thing with my marathon time being what it is is that I have family members who I'm confident I could beat in anything from 5k to half marathon that have better marathon PRs than me. I know how stupid it is to care about that, but that doesn't make me immune from caring about stupid stuff.

Why? No Subway or McDonald's along the course? How did people run marathons, let alone fast marathons, before pre-packaged corn syrup and caffeine shooters came on the market three decades ago?

They must have eaten something right? My dad's cousin who's one of those family members I described used gummy worms in his race.

The only person who cares about your marathon time is you. No one else cares. Absolutely no one.

So relax and learn to enjoy the process. The actual race result will change nothing about your life. Nothing.

I thought you were a former semi-pro when I read your post. If you are only 2.5 years into running it is not weird AT ALL that you struggle a lot more with fulls. A marathon requires a lot more aerobic work and strengthening of your legs to perform at an equal level as shorter distances initially(Especially a 5k..). Reaching your max potential at 5k or half would require the same effort of course, but at your level underperforming at a marathon compared to other distances is not weird at all.

Also, at your level, the 4 months of marathon training was probably really good in terms of training towards the shorter distances as well, so you didn't really waste any time at all training wise either.

A former semi pro with a 2 minute gap and talking about building mileage to something decent for marathon training?

I make up a conversion from shorter races and say that's what my marathon time is.

You trained a whole 4 months? How much and how long did you study marathons before you started training for that 'whole 4 months'? You should have studied long running (marathons - endurance running) for 4 months before even starting your physical training. It's a lot more than just going out there and running. I know at least 100 marathon runners that trained (educating themselves along the way) for a year before their first full marathon. I personally studied running and marathons for 6 months before I even set my first foot on the pavement for physical training and ran my first marathon 12 months later.

Have you ever had a physical therapist tell where your week points are that would inhibit your endurance running? Did you study nutrition and hydration - I mean really study it? Did you study the affects or your different wearables - shoes, socks, shirts, shorts, hats, etc.? Did you arm yourself with the correct electronics to best gauge your training progression? Did you hook up with a respected marathon training program (fitting your level of competence)? Do you have the right 'WHY' in your head? Are you able to 'move on' when you don't nail your expectations or do you continue to sit and cry like you are now?

1:23 and 2:52

Closest comparison to OP would be the year I ran 1:25 and 2:57 a month apart.

For most runners I know, the old rule of thumb -- half marathon time x 2 plus 10 -- produces a result pretty close to marathon time.