And looked down one as far as I could To where it bent in the undergrowth (4-5)
The speaker is looking down the path, imagining where it could take him in the future. This is the first path he looks down, and he spends a lot of time examining it before he chooses to take the other. You'd think he would see something that he doesn't like before he decides to switch, but he can't even see anything past the bend in the road. So he just arbitrarily decided which path to take. As it is with the future, he could only see a little bit down the road. Maybe he'd planned to take this road, and that's why he studied it so much, but after a minute of reflection, he changes his mind, and opts for a different future.
Oh, I kept the first for another day! (13)
So now that our speaker has walked away from one future and into another, he's pining for the one he passed up. He is planning to come back and take the other path another day. It's like deciding not to eat desert and saving it for tomorrow – except way more important.
Yet knowing how way leads on to way, I doubted if I should ever come back. (14-15)
Despite the speaker's hopes that he'll be able to take the path he passed up on another day, he realizes that the nature of paths, and the future, is that he'll probably get too carried away on the path that he's on to come back and take the one he didn't. This shows that the speaker might be regretting his choice, but, even more so, that he's bemoaning the nature of choices.