Nether portal have a special algorithm to link.
When you cross a portal in the overworld, it computes the nether exit by dividing the coordinates by 8 (X and Z only). Then the game look for the closest already existing portal within 128 block radius around that position. If there is one, you have your exit. If not, a portal is created.
Reversely, when comming from a nether portal, the game multiplies the coordinates by 8 and look for the closest portal within 1024 block radius around the computed coordinates.
So for the long travel idea you would need to do as this:
- Write OW coordinates. Spawn your portal and go to the nether.
- You should create a new portal (first one). Mark the location and check it is around the coordinates of your first OW portal divided by 8.
- Destruct this portal and build it as far as you can from its original location in the direction of travel, that is to say at the edge of the 128 block radius.
- Write the N coordinates and compute the equivalent OW coordinates
In this configuration, the first portal A links to B and B to A by the means of the "closest active portal" rule.
- Add a portal in the OW in the direction of travel that is closest to the last OW coordinates you have written.
You have now the new portal C linked to B (closest active portal in the nether) B to C (closest active portal in OW) and A to B (closest active portal in the nether). You can fast travel A -> B -> C now.
If you need a longer route:
- compute the nether coordinates equivalent to the C portal.
- Build a D nether portal closer to this point than portal B in the direction of the travel and repeat the operations.
The key here is to exploit the "closest" search algorithm. Do not use original spawned portals cause they are centred in the radius range.
Warning, do not build new nether portal after this in the influence zone of your fast travel road. You could easily messed things up.