10km is not outlandish, especially if you can use more than one pair. In telecomms, equipment known (to me, in the UK) as CWSS or HDSL (equipment made by ECI Telecom and Fujitsu respectively) was used as the standard solution in the national network to provide standard 2Mbit links (EG for ISDN30 phone systems) or 1Mbit data over short distances on single-pair. It was originally designed as a quick-fix whilst waiting for fibre to be installed but proved good/cheap enough (probably more the latter) to become a permanent fixture.
The kit uses 2 pairs as standard for 2Mbit, 3 pairs for long lines or poor/noisy cables, or 1 pair for short runs at reduced speeds of 1Mbit.
For slower speeds (<1Mbit), very old (1970's - 1980's) kit called Kilostream was used, although I doubt you'd find anything much about that of any use.
Base-rate (64/128k) single pair ISDN is very robust and the kit is not complicated, I don't know how hard it would be to find the "serving" end though. The customer end (NTE) are common and disposable, I've probably got an old one laying round you could have for the price of postage.
All this is constant rate bi-directional, so not subject to speed fluctuation or asymmetric data rates like DSL.
Unfortunately all these technologies are designed to be run from a serving exchange, so the customer end equipment is very available but the other end really isn't, and is usually designed to slot into a 19" rack shelf serving multiple of customers.
There will be international standards for all this kit as nothing goes into the telecomms network without it. Whether you can find them, and sustain the will to live whilst reading through them, is another matter.