I rode 5 hour centuries in my 30s but no way in my 70s and I am pretty fit (Typically in the top 5% of my age group on Strava segments). With a group running a pace line it might be possible, but solo, it aint’ happening. I did an 86 miler in May and riding time was 5.5 hours with 3,300’ of climbing, so 6 hours might be possible.