Calves lock up at 18km on every long run but never on anything shorter Training

Second thing to look at is the jump. Going 14 to 22 in seven weeks is fine on paper but if the long run is a much bigger share of your weekly volume than it used to be, the calves are just doing an unusual amount of work in one go. What is your weekly total, and what percentage is the long run?

Shoes at 400km with a marathon build coming are worth rotating rather than replacing outright. Two pairs alternating changes the loading slightly on each run and is genuinely easier on calves than one pair everywhere.

The threshold being so consistent points at fuelling and duration rather than a structural problem. Under two hours you can get away with nothing on a 16km, but by 18 to 22km you are past two hours and running on empty, and tired calves cramp. Try taking something with carbohydrate and sodium from 45 minutes onward, roughly every 30 to 40 minutes after that, on every long run rather than only the very long ones. That single change fixed the identical pattern for me and for two people in my club.

If it keeps happening after you have fixed fuel and volume, get it looked at by a physio rather than diagnosing it here. Calf tightness that is that reproducible sometimes traces back to something further up the chain, and that is not a thing anyone can see from a text post.