Enzo is proving his own selflessness here by indicating his desire to make his own death as easy as possible for Denny, since he's already been through so much.

Enzo visualizes distance as a metaphor for death, which he runs to, as quickly and freely as he can. Huh, and we thought we'd be all out of tears by now.

This is one of the last things that Denny says to Enzo during his last minutes on earth, before he lets Enzo know that it's okay for him to go, to be free from his life and his dog body and his pain. It's the simplest proclamation of love in the book, and the most bittersweet. No, we're not crying. You're crying. Now hand us that tissue box.