Whitehaven to Sunderland twice.
Once with girlfriend (now pregnant wife so it can't have been that bad!) who tripled the length of her longest ride for the first day and had doubled her total mileage on a bike before we finished. Unsupported over 5 days, me with a trailer and most of the kit, her with a pannier rack and various bits increasing in weight by the end. She was not unusually fit before it either.
Second time in one hit, about 19 hours in absolutely atrocious weather, taking all the off road options but were supported (probably stopped too often and for too long but next time we will nail it 🙂 ) I was not very fit and had only completed one 100k Merida marathon before that, just, but sheer bloody mindedness got me through.
Not an insane idea, actually a fantastic idea, seeing 100 miles roll over on the bike computer was an awesome feeling (only being 2 thirds done was a less awesome feeling) but as I have mentioned before, riding through the night and through dawn is possibly the best thing I have ever done.
Talk about uplifting (the rain even stopped for about an hour).