Nothing suggests where the poet is journeying in this and the following sonnets. All that is known is that the poet is on an unnamed journey away from the young man. The poet's allusion to solitude has no definite time frame, and the journey may be brief. However, the youth is the standard against whom the poet measures everything, so it is not surprising when the poet says, "Thus far the miles are measured from thy friend."
The poet draws an analogy between himself and the beast on which he rides: "The beast that bears me, tired with my woe, / Plods dully on, to bear that weight in me," as though the non-physical weight of the poet's sadness factors into the burden that the beast must carry. Similarly, the groan that the animal makes prompts the poet to recall his own sad state in traveling farther away from the youth: "For that same groan doth put this in my mind: / My grief lies onward and my joy behind." Here, "onward" means physically forward, but it also means into the future. Because this future doesn't involve the young man, the poet is grieved. Likewise, "behind" means from where the poet physically has traveled, but it also means the past, which was joyful because the poet had the affections of the youth.