Answer: 1) old.
Explanation: In the given sentence: "My dog and I would run three miles every day when he was young, but we've shortened the distance to one mile now that he is getting up there in years" the meaning of the phrase "getting up there in years" is "old", the speaker is saying that his dog and he used to run three miles every day "when he was young" but now they can't run that distance because the dog is old.
