Explain lines 45 through 66 of the poem "The Last Ride Together" by Robert Browning. The explanation should cover the lines of the stanza beginning "Fail I alone, in words and deeds?" through "My riding is better, by their leave."

The lines you have described constitute the fifth and sixth stanzas of Robert Browning's "The Last Ride Together." The poem itself consists of ten stanzas, each 11 lines long. The poem is written in iambic tetrameter, with each stanza rhymed AABBCDDEEEC.
The poem is a dramatic monologue narrated in the first person. In earlier stanzas we have discovered that the narrator is in love with a woman who is not in love with him and that she has effectively ended the relationship. Nonetheless, she agrees to his request that they go riding together one last time. The situation of these two stanzas is that the poet and the woman he loves unrequitedly are now riding together. There is no detail regarding where they are riding, the precise time of day, the weather, or the landscape. The lack of detail is deliberate, as the narrator wants to emphasize the emotion and spiritual aspects of the situation, which he considers more important than material details.
The fifth stanza begins with the following:

Fail I alone, in words and deeds?
Why, all men strive and who succeeds?

The narrator worries he has "failed" at love and wonders if he is unique in being a failure or whether, in fact, all people in some way feel that they fail to achieve the ideals towards which they strive. These lines introduce a theme common in Browning, sometimes called his doctrine of imperfection, the belief that it is our imperfections and the way they make us strive towards self-improvement that are characteristic of our humanity. Without imperfection and failure, we would lack ambition and goals.
The poem takes a metaphysical turn as the narrator contemplates traveling past unknown regions and cities. On a literal level, this could not be accomplished in a single ride; instead, the narrator is talking about his mental state rather than the physical journey, suggesting that as he rides he thinks about humanity as a whole, not just his immediate situation. He thinks that most people have this awareness that their actual deeds never fully measure up to their hopes and dreams. Even if he has not found eternal love and is disappointed with how things turned out, he takes solace in his ability to savor the moment of the ride, suggesting that even as we strive towards goals we cannot attain, we will achieve moments of joy that would not have existed without that striving; thus, rather than be disappointed in our failures, we should appreciate our positive experiences.
The sixth stanzas expands on this theme. The narrator considers that statesmen and soldiers also never achieve their grand goals, and that the greatest heroes end up dead, with only an impressive grave to show for their efforts. As a result, he feels increasingly appreciative of how—at least in his attempt to attain love—he has achieved this wonderful ride in which he can feel joy at contemplating his beloved and what he has enjoyed of the relationship.