Marathon dream is gone for good, If i even mention it herself will chuckle me, Three years running is too much of a coincidence,
My body is telling me its time to forget it,
Not at all. You've a good outlook and you're right to shift focus to shorter stuff. You'll smack that half PB sideways, but don't be afraid to set some shorter targets, a 20min 5k is a good one and mostly achievable. if your PB there is about 24mins (at a guess) shoot for 23 next, then 22 etc with the aim of 20 once you've jumped the other barriers
Definely take some time to figure out what's up that you sre breaking down during your marathon buildups. I dont know you or your training or your training history but I'll bet you some small changes will reap dividends
Based on your posts you seemed to go from walk/running 10 miles to running 16 and 18 mile.long runs over a very short period. The stress fracture would indicate you ramped it up too quickly. Go onto Garmin connect and write down your weekly milage totals for the last 12 months. it'll take 5 mins. I guarantee you made a big jump about 6 weeks ago which you weren't ready for.
When you ran your 18 miler around Ballyphehane how long did it take? based on your half PB I'd guess >3 hours , did you fuel? if so how much. You'll be well into birning liver glycogen when you're on your feet that long, did you take a recovery drink after 3:1 carb protein. not just protein It'll do you the world of good for anything longer than 90mins or so.
If you're about 50 years old, Some protein 2 hours before bed is good idea. Maybe strength work
If you have dirty calves after a run you're likely wonkey at your feet (maybe pronating) do your "A" drills routinely.
Consider getting some milage cross trainingm Bike, eleptical etc. your legs need the roads but your heart doesn't give a shit
You're not likely to be made of different stuff that makes you especially fragile. you'll run your marathon just not this year is all