I find it easier just to play the parts in, in real time, than to go back and edit every note for velocity, precise timing, etc.
By playing in real time, if I haven't practiced it enough beforehand, I'll play a few wrong notes, but I don't let that bother me, as long as the timing and dynamics are there, I can fix a few notes in the piano roll editor.
I've never attempted anything this huge, though. It seems to be a monumental task.
Mostly I make my own backing tracks for my duo http://www.s-cats.com and make aftermarket styles for Band-in-a-Box @ http://www.nortonmusic.com
I do this by first recording the drums, then the bass, and layering other parts on top. I record everything in real time, that way I get subtle timing and dynamic expression. When the drums are down, all the other comp parts have to respect the groove and either sync with it or contrast it. Fortunately, I play drums, bass, sax, wind MIDI controller, guitar, flute, and keyboards. Life is too short to play only one instrument.
The longest things I've recorded are 20-minute medleys that we used to play during dinner sets. I did two in light swing rhythm and one more in a rhumba and bossa rhythm. I did these by recording each song separately and then pasting them together in Master Tracks Pro.
But there is more than one correct way to make music. My way is best for me, and will be until I learn a better way. YMMV
Notes ♫