I'm also not a big fan of night riding, so I've limited myself to 400k on my single. I can easily do a flat 400 in daylight, and a mountainous one with only an hour of darkness. On a summer 600, my approach would be to do the 400 part mostly in daylight, sleep until just before dawn, and get 'er done before the limit. Same theory for longer rides, though that's just theory for me, having not done them, but that's how my riding buddies approach them in their old age. And I'm not even fast, just well trained and on a good bike. You only have to average about 14 on elapsed time, which is about 16 in the saddle. So that's another suggestion: research it and train it up.