Yep, it’s doable. I’ve done it a few times. I’ve done black Hub to Hub, then red but skipped Falla Brae at the end (as it is a repeat of the Black finish), then a full blue to finish. Doing that is about 55k I think; took me about 5 hours including a quick pitstop at the Hub. You could slot the Buzzard’s Nest Green in there too; the one from the Peel is a repeat of other stuff you will do anyway.
Adding Innerleithen Red plus the ~10k each way on the road would probably be beyond me in winter.
It would be properly dark about 4 but it gets very dark under the thick canopy on overcast days a lot earlier than that.