When I started audax a few years ago I used to spend a lot of time mile-watching and counting down the distance to the next control. I realised that what I was doing was, in effect, spending most of the ride looking forward to finishing rather than enjoying the ride. Hiding the computer behind the routesheet holder helped a bit, but the psychological breakthrough came on my first 300 when I decided that rather than thinking about the distance I would think instead about enjoying a full day out on the bike.
I have overcome my arewenearlythereyet-itis by thinking of a 200 as a day out, a 300 is a long day (dawn till dusk), 400 is a night ride - with a ride to the start and from the finish, and 600 is a weekend on the bike.
I had real doubts about finishing the 600 as the route passed my house just after the mid-point control and I knew that I would have motivation problems leaving the house at 1am to ride another 300k. This was where riding with a group (including clubmates who I'd ridden the other SR events with) really helped. The first mile or so was tough, but once our group was together again I had no more doubts about finishing.