I think my longest was the Yorkshire 3 peaks, which I think is 24 miles. I remember at the top of Ingleborough sitting at the summit cairn and thinking that there was absolutely no way I could do the last 5 miles back to Horton in Ribblesdale. But as that was where the car was, there wasn't much choice.I was 60 then, and I have done a few 20+ milers since, but not with so much up and down in them. I am another one who reckons I can walk further now than I could when I was younger, though possibly not so fast.
On the other hand, I prefer thinking in terms of 12-15 miles a day. I need a reason for long days, accommodation, bus back to the car, finding myself not precisely where I intended to be or something. I am definitely not enjoying it after about 18 miles, and as far as I am concerned enjoyment is the reason I am out walking. Though I have noticed, however long or short the day is, when I realise I only have another mile or so to go I feel suddenly very very tired and wonder how I am going to manage the last bit!