Haven't done a lot of these recently but back in the days of BVW70s and BVE 910s (tape editing) I'd determine the bpm required then lay down a beat track on 3&4 then crossfade channels 1 & 2 in same manner as a DJ used to switch between turntables. You need to group your music clips in such a way that the extro of clip A is similar to the intro of clip B then by matching the beats, you will gets seamless audio transitions, with the beat track filling in holes.

BTW: I had a part time job as a Club DJ back in the 80s and had to keep the people dancing non-stop for 45 minutes or so then slow down a bit so they could go replenish their watered-down drinks and overpriced beer then "give 'er" again for another 45:00. You should have no trouble building a 50 minute audio track.

When switching mood you might want to use some sort of short (00:10 max) transition like off Gene Michaels Tags & Stingers (for example) to slow the pace down but when picking it up use a fast clip with a longish instro and fade it in so that the fiirst crescendo occurs as the next block of visuals establishes.

Peter