I do the great north bike ride every year,
Event and that's around 55 miles and it's done in a morning. Usually to train for it I do a 49.7 Mile route into the moors which is pretty much just steady climbing on about 70% dirt track with the rest being paved. On flat I can get around 80 miles before I get massively bored. Hills just make cycling a lot more interesting. Really do need to get back in shape for the GNBR though.