THE cast of the latest Agatha Christie whodunnit to take to the Theatre Royal stage will be fighting fit by the end of the run if TV star Cathy Shipton has her own way.

The actress best known for playing Lisa "Duffy" Duffin in 109 episodes of the BBC drama Casualty has completed four London marathons, clocking up an impressive best time of three hours 39 minutes. She is a trained aerobics instructor and has written a book with former top athlete Liz McColgan which has become the Spider's Web cast's fitness handbook.

"All the lads in the show have taken up running. Denis Lill's wife has taken it up too. She said: 'I can't do this'. And I said, 'Start at five minutes and build it up. Here's my number – I'll be your mentor'."

Cathy, now 52, didn't take up running until her mid-30s but now you can't stop her.

"I had a letter from a cancer charity offering me a London marathon place for 2010 and I got a bit twitchy saying 'can I do it again?' I can prepare for it in 12 weeks and I can tick over until then.

"I finish Spider's Web in December and thought I could start my programme in January but I wouldn't be able to work, not much anyway. I opened a show in the West End and trained at the same time once but I was at the end of my programme then. I did the marathon on the Sunday and played on the Monday."

That all-round fitness is useful when on a long theatre run and Spider's Web, the fourth thriller from Bill Kenwright's Agatha Christie Theatre Company, is proving a very successful show.

"Here we are in Cambridge and the marketing people are grinning from ear to ear," says Cathy. "One because it's selling so well and two because they enjoy it so much. There's so much humour in it. Bill Kenwright came and he was smiling as well and saying 'how am I going to top this one?'

"The Agatha Christie trust is very cagey at letting rights go. They have been exploited on films and TV but Bill has been very clever in getting sole rights to bring them professionally to the stage.

"His vision of them is that he will maintain the standard set by TV and film and you can see that in the work. We are really benefiting from being fourth out as there's now a real fanbase for the shows. This tour ends on July 18 and starts again on August 17 and runs until December.

"It has been brought forward a month because it's selling so well. Do you know more people see live theatre a week than go and watch football? These are cash-stretched times so it makes me want to do my very best job and I know people are being thoroughly entertained before going back to Newsnight. I'm happy to do that as a lot of my work has had a message or something like that."

That was the case with Casualty, the show which Cathy will always be associated with. Something she can never change.

"Nor would I want to," she says. "It was a real TV break for me. At that point I was on the verge of giving up as it had been around five years since drama school and I had said 'five years or out'. I had done the National and it had gone quiet, been in the West End and it had gone quiet and I had taught aerobics in between. I thought I should go back to university and change my plans but Casualty landed in my lap.

"I was proud to be part of that show. The letters you receive are marvellous. They say things like, 'the way you handled that person in that episode happened to me' or 'that piece of advice you gave really helped me'. I tell them it was the script and that I went to drama school and I'm not really a nurse. But it's so nice when people say 'I'm a nurse because of you'. It's remarkable. I'm proud of that show."

SPIDER'S WEB

WHERE: Theatre Royal, Nottingham.

WHEN: Today until June 27, 7.30pm nightly, with 2pm matinee on Wednesday and 2.30pm matinee on Saturday.

TICKETS: £9- £24.50 (concessions available).

BOX OFFICE: 0115 989 5555.