This month, white-haired and swollen-ankled, she will at last achieve her ambition with the publication of A Nest of Magpies. At almost 80 she is thought to be Britain's oldest debut novelist.
Flattering reviews of her love story, set in the Fenlands during the 1960s, led publishers Michael Joseph to reprint before publication, and Penguin plans to publish the paperback next year. Meanwhile, in her small, book-lined study, overlooking Ely Cathedral, Mrs Marshall is now tapping out a sequel on her computer.
Sitting at her desk, between the walking-stick in one corner and exercise bike in another, she is completely relaxed at the prospect of a new career so late in life. The book is only the latest in a series of achievements that expose all the nonsenses of ageism.
'I have come late to everything,' laughs Mrs Marshall, looking nearer 60 than 80. 'When I was 18 we were in the midst of the Depression. My teachers expected me to go to university, but I had to go to work to save the farm. I was 47 before I eventually went to university. People said I would never manage, that my memory would have gone. But I loved it. I did my tripos in two rather than three years and came out with a 2:1. We don't have to put our mop caps on at 45 like our grandmothers did.'
Cambridge University was just the beginning. Before graduating, Mrs Marshall wrote a book about her 25 years as the only teacher in a one-classroom country school. It became required reading for teachers. In her mid-fifties, while her peers were contemplating gardening, golf and comfy fireside chairs, Mrs Marshall was carving out a career as university don, Fenland historian and writer of school books. In the late 1960s she devised Picture Box, the popular Granada Television schools programme, which she worked on until two years ago.
Her mid-fifties also brought love. When she fell for Ewart Oakeshott, she was long-divorced, he still married. There followed a year of conscience wrestling. It was the Swinging Sixties but 'divorce was still a dirty word' in many circles.
The relationship is relived in her novel, which tells of a couple in their fifties who break the established moral code when they fall hopelessly in love. The questioning of conventional morality takes place against the gradual collapse of the Fenland village's traditional social order.
The book combines Mrs Marshall's passion for a lost way of life with her love for Mr Oakeshott. She admits that he is William, the hero of Magpies.
No need to give away the ending, but today it is Mr Oakeshott who provides coffee and biscuits for Mrs Marshall's visitors. Now 77, he is a world authority on medieval swords, and suitably chivalrous. The couple are clearly devoted. 'William is a lovely character. All the women who have read the book fall in love with him,' said Mrs Marshall. Mr Oakeshott, a tiny, round man with wisps of white hair, counters: 'I recognise events and characteristics, but William is tall and slim with curly blond hair. I wish I looked like him.' He may not be as handsome as William but with his New Age-style choker and rings, he is as disarmingly youthful as his partner.
Mrs Marshall sees the irony in her deep regret for the passing of the old village order. Had she and Mr Oakeshott, like the heroes in her book, not eventually rejected the rules upon which that order depended, they would not be together.
But the tension is still there. Mrs Marshall insists that 'love with a capital L' cannot in the end be fettered by conventional morality, but she and her partner have never married. She admits to a lingering guilt and is still concerned about the feelings of his wife, now 83.
She links social change with the personal. 'What we have also lost is something the French call tendresse. It is an all-encompassing, tender love for which there is no word in English. Today there is so much concentration on sex from a biological point of view. In the book I was determined to put tendresse into a relationship which would last until the people were 80.'
In A Nest of Magpies, William does eventually bed his conscious-racked love, but there is no graphic detail. Mrs Marshall did put some in, 'because I thought they would want that'. She is amused that the publishers took it out.