There is nothing magical about a marathon in terms of training relative to any other running distance. This is why you can reliably estimate your marathon time from a half or 10k. In reality even a 5k provides a decent basis for estimating a marathon in the same way a 20min FTP test is a good indicator of cycling performance over much longer intervals. Run the numbers through something like this: https://www.runnersworld.com/...race-time-predictor/
and see where you come.
If your goal is way off the race estimator then you need to tweak your training. If everything looks about right there is no need to make big changes.
I, and nearly all hardened experienced marathon runners, definitely disagree with you greatly on this one.
I would actually strongly say that for most 'AG runners' (nonelite), 5ks-HMs are almost interchangeable and accomplishable just by modifying pace and race strategy, while using similar training volume and intensity.
The MARATHON, however, is a totally different beast. At 26.2 miles, you better have lots of high mileage training in your legs to keep the distance - the VAST majority of folks underperform their 'calculator' estimate by 20+ minutes the first time out, and 10+ minutes on subsequent attempts.
Even if you can run a 70.3 at a good clip, and feel fine the next day, you will be in for a monster rude awakening when you show up to marathon day, expect to run an adjusted 'calculator' pace based upon even a recent standalone HM performance, and then watch in horror as your legs cramp full out at mile 20.
For the OP, I def think you can FINISH the marathon on 70.3 training, and the 'respectable' component is totally subjective and up to you. If you're content with finishing +30 mins slower than your calculator prediction time, you'd likely be fine. For <5 or even <10 mins to your marathon calculator time, highly unlikely, and you may very well get a short-term injury during the race that will sideline you for a month as a result.
I will note that here on ST, there are a surprising number of fast people that can run sub 3:15s on <25mpw. (There is some guy who runs sub-3 on <15mpw!) These guys are the EXCEPTION to the rule. I've been on plenty of marathon forums in the past, and nearly nobody pulls this off (although mannnnny people try.) And it's not just my opinion - there will be virtually no establish triathlon coaches who will claim to be able to reliably get you to a strong marathon performance of pure triathlon training - they would actually dissaude you from the marathon until you can commit to it.