"Of course people think I'm mad, including my own family," says the neatly dressed and coiffured Anderson who, as we chat in her Kent home, is barely recognisable from the pictures of her when competing. "But I do like a challenge."

Her "challenge" is to race non-stop 246km (153 miles) from Athens to Sparta, following the route that Athenian messenger Pheidippides took when he ran with a message from the king to his generals in 490BC during the Battle of Marathon. (Marathon to Athens is about 26 miles, hence the modern marathon length.)

But that's for wimps; after completing Spartathlon (and usually only a third of the 330-odd competitors do) Anderson, who ran it in 2011, will take a short rest, then run back to Athens. If she succeeds she will be only the second person to achieve the double, and the first woman.

Clearly ultra running is not for the faint-hearted and takes gruelling training. Anderson runs anything from 12 to 20 hours a week, covering 70 to 100 miles in total, and takes only one day off. But she does permit herself a rest of a week, or even two, after an event like Spartathlon.

Ultra running, Pheidippides excepted, is a relatively recent sport, although some modern races go back half a century. Ultras start from 30 miles/50km but the distances are frequently much greater, and they can be non-stop or staged over several days. At the elite end of the sport there are strict time cut-offs at each stage when slower runners are eliminated; the maximum time allowed in Spartathlon is 36 hours, and participants have to complete a 100km or 200km race in a qualifying time in order to take part.

These are serious races for serious runners and, unlike the London Marathon or the Great North Run, nobody will be running dressed as a farmyard animal or Mickey Mouse, although many runners do compete for charity. In the UK alone there are about 200 ultra races each year, ranging from 30-milers up to the 268-mile Spine Race across the Pennine Way (in January, just to make it interesting). There's no prize money in the sport, but some sponsorship supports a few professionals, including Britain's Lizzie Hawker, the fastest woman at 2012's Spartathlon.

Anderson started in 10 km and half-marathons in 1999 and did her first ultra two years later, and has noticed a rapid growth in the sport, as well as a changing age profile. "When I started there were a lot of thirty and fortysomethings competing who were stepping up from marathons, but the sport has exploded in the past five years and a lot of younger people have taken it up," she says.

Anderson, who grew up in an army family in Edinburgh, was 36 when she started running. She mentioned to a friend that she would like slim legs. Her friend said running would do the trick, and Anderson was soon hooked. Remarkably she had battled to overcome anorexia, which she suffered from for 15 years. It started when she was 14 and at boarding school – "Yes, classic, I know".

Despite once seeing a psychiatrist at her mother's behest – "He was trying to blame my father and I knew it was nothing to do with him, so I never went back" – Anderson doesn't come across as one for major introspection, although she says: "I've probably given up one obsession for another. Actually I think most people who do ultra running have obsessive personalities. You probably have to."

Does she worry that she may damage her body by doing the sport? "Well even if I did need a hip or knee replacement, at least I can say I've jolly well enjoyed what I've done to cause it. But I listen to what my osteopath tells me and if he says I can't run for a few days, then I don't." But she doesn't put her feet up. "I can always do cross-training instead."

She says running has given her self-belief and acknowledges it's a selfish pleasure. "When I first started running I really enjoyed doing something that didn't involve my children, my husband, or running the house. It was purely for me."

But when her husband, Tim, said he felt left out, she suggested he crewed at her next big event, a John O'Groats to Land's End world-record run in 2008, and he will be at Spartathlon with her "lucky mascot" Becky Healey. Support crews may not aid competitors while they are running, although they can leave food and drink at checkpoints.

Talking of food and drink, what does she do is she needs a bathroom break? Anderson hoots with laughter. "You become pretty adept at finding a convenient bush," she says, "and I'm sure there'll be plenty of vine leaves nearby should I need them."