I don't think telling myself I'm halfway there would work, although telling myself it's only 50 miles might work. If things are really going badly, I just pick a point along the course where it's the same distance as my commute, 7 miles. Or just 20 miles to the next stop. Or just 100km to the end, I've done that many times. On a 1200km ride, I have thought, "well, it's just 200km to the end, short ride."