They never spoke about it. The day they finally met. When he came to her building to search her tenants flat. When their eyes met and connected, they knew. Soulmates. Both were aware how unusual they were, their massive age difference. Much care was given so they did not touch, for contact with each other's flesh would solidify their bond.
After the first meeting, they kept in touch. Going to see a film together, the odd cup of tea, they would not be able to be out of each other's life. Even of enduring the pain of not bonding, they would still seek out companionship of others, especially after his divorce. They even thought Sherlock had never deduced it, but he had, keeping the information to himself, not out of respect, but because they made him uncomfortable.
He would visit her often after Sherlock "died", taking care she wasn't lonely, for he knew how fond she was of him. They had a true adoration of each other.
Years later, when Greg walked into hospice care to visit Mrs. Hudson, he was told she didn't have long left. He sat next to her and she looked at him for the last time. With tears streaming down his face, he kissed her at last.
Wait for me, my love.
And then she left.