He's actually surprisingly chirpy, even though we are meeting to discuss the most morbid of subjects, death.
He talks freely about the topic of his latest book, The Welcome Visitor, co written with his doctor, Sarah Jarvis. The book was inspired by the lingering, undignified death of Humphrys' father, who simply didn't want to carry on living after his wife died in 1990.
Humphrys senior turned to drink, downing at least a bottle of whisky a day. He eventually collapsed but was brought back from the brink by doctors and, in his final years, underwent a personality change as dementia set in.
Humphrys' father became vindictive and vicious towards his loved ones and spent his last days in a mental institution for the elderly in Cardiff, growing weaker and longing for death. Eventually he stopped eating and drinking - and nature took its course. He was 91 and had outlived his wife by 13 years, if you can call that 'living'.
"It was ghastly," Humphrys recalls.
"It was hideous for him and for everybody involved with him. It can destroy your relationship with somebody you've loved your whole life. You didn't want to see him because you knew how unpleasant it would be. He wasn't the person that we knew."
Afterwards, Humphrys wrote a column about his father's death and received more than 1,000 letters in response.
"There's a huge number of people who are suffering in this country alone. There are about 800,000 people with dementia, that would mean by the time you allowed for all their loved ones you're probably talking about two to three million people who don't know what to do or how to sort their thoughts out on it all."
The main point of the book, he says, is to allow people to control their death as they have their life. He argues strongly for the legalisation of assisted suicide.
"My point is that if something ghastly does happen to you then you should have the ability to put an end to your life in conditions that are acceptable. You shouldn't have to traipse off to bloody Switzerland to do it."
Last year, Humphrys' younger brother Rob died from lung cancer in his 50s, just seven weeks after being diagnosed. Humphrys still feels guilty about the whole episode.
"The reason that I feel guilty, as everybody does in this situation, is that I wanted to be able to say, 'If it gets unbearable or you decide you want to put an end to it, then we will be able to do that', but we couldn't'."
Humphrys insists it shouldn't have to be that way. He says his late ex-wife, Edna, a former nurse, had a much better death, although she was only in her early 50s and suffered pancreatic cancer.
"She was able to control the pain. My daughter gave up her job to care for her and in the last few months of her life she got a lot done. She wanted to put things in order and she did all that and was under the control of an absolutely first-rate hospice and was getting regular visits from the Macmillan nurses.
"When the end was very close, they gave her as much morphine as she needed and she died without having been in really dreadful pain."
Humphrys grew up in Cardiff, the son of a French polisher. He left school at 15 to work on the Penarth Times and then The Western Mail before joining the BBC.
For 10 years, he owned an organic dairy farm in Wales, an experience that drove him to comment that "taking on a Cabinet minister is nothing to handling three tons of kicking cow".
Despite the morbid talk, he doesn't brood endlessly about his own mortality. At 65, he is wearing well, but then late fatherhood - he has an eight-year-old son, Owen, with his former partner Valerie Sanderson - is keeping him fit.
"It knackers you and also keeps you young. I kick a football around with him, or he comes running with me or he cycles while I run and all that kind of thing.
"I feel about 20 except when I look in the mirror. But would I have plastic surgery? You've got to be joking!"
All those early mornings on the Today programme don't seem to have caught up with him yet and he admits that he has very little free time.
"I've always kept busy. I don't really know any other way of living," he says.
"I will probably go on till I drop. I get up at two minutes to four, which means I can be in the office by 16 minutes past four and I go to bed early, at nine o'clock.
"I hate gyms, but I jog around the park and wherever I have to be I go for a little run in the mornings and some evenings. I eat sensibly, I don't drink much and I gave up smoking years ago - I'm a right little goody two-shoes."
He has a place in Greece where he goes to recharge his batteries and another in Wales, but London remains his permanent base.
During his 22 years in the hot seat of the Today programme, he has grilled the best - and the worst - of politicians. Yet he says he has a good deal of respect for most of them.
"I don't know that I've ever interviewed a crooked politician - well, I can think of one from about 30 years ago - but like any section of the population, some are more strict with their moral approach than others. By and large most of them do a decent job."
Humphries doesn't consider himself an aggressive interviewer although he agrees his style has changed over the years.
"I consider myself to be a persistent interviewer and if I don't get an answer to a reasonable question I persist. I probably have a sense of humour that I didn't have when I was starting out and wanted to show that I was a tough guy when I was the new boy on the programme."
He rarely gets nervous before an interview, yet always feels he could have done better afterwards.
"I am hard on myself all the time. I don't think I've ever done an interview where I've said, 'Ah yes, that was exactly how I wanted it to go'.
"There are always things wrong and you always wish you could do the interview again - well I do, anyway."
The Welcome Visitor by John Humphrys with Dr Sarah Jarvis is published by Hodder & Stoughton, priced £16.99. Available now.