Used to regularly play 5/6 hour sets, always thought the best approach was stringing together lots of little pre planed bits.
I'm in this camp. When I discover two tracks that 'belong' together, I'll make a note of it. Doesn't necessarily mean I'll play them together in a live setting as the vibe might not lend itself to it, but I would always have that to fall back on.
Take a certain Welsh DJ for instance. Remember when you came home from a party waxing lyrical about how he played this tune with that; perhaps the basslines swapped at just the right time, or the intros/outros met at just the right point...you think they were coincidences? It was the result of hours of practice. People turning their noses up at pre-planned mixes always baffled me.
Finding two records together that when played together make a third - that's what always floated my boat. Hell, I bought decks after listening to Ralph Lawson mix two particular records together on an E-Mix, and I wanted to replicate that mix.
Nowadays the Welshman doesn't bother with it anymore, and the sets are significantly poorer for it - it was his USP.