If you're a hillwalker, you may even have given them a cheery wave or exchanged pleasantries in one of Scotland's most scenic spots. For the past 23 years, they have been diligently tramping up hill and through glen on their mission to produce walking guides for every corner of Scotland. Now they are celebrating a mission accomplished.
Dubbed "the Scottish Wainwright", 73-year-old Mary Welsh and her friend Christine Isherwood, 65, have walked more than 6000 miles together; the distance from Glasgow to Rio de Janeiro.
When we meet the pair are heady from the publication of the final books in the series. Our meeting point is Carstramon Wood near the village of Gatehouse of Fleet, the starting point of one of the walks from their Dumfries and Galloway edition.
Welsh, a biologist, former school-teacher and mother-of-four, cuts a dashing figure amongst the predominantly utilitarian-dressed walkers. She is all in blue with a matching headband which cuts across perfectly styled curls. Her face is fully made up and she is clutching two walking sticks. "I think I must be the only person on the hills wearing bright pink lipstick. I never go without it," she says with a twinkle. Isherwood, a biologist, illustrator and mother-of-three, met Welsh when she taught Isherwood's children.
As we make our way through the 200-year-old wood, the bluebells are still out. For two ladies based in the Lake District, it seems unusual that the bulk of their guides are for walks in Scotland. "I used to youth hostel in Scotland at 16 and 17, and always fell in love with the wardens and that was what started it all," explains Welsh. "Much later, after we had lived in London, my husband Tom and I moved to the lakes and Scotland was so much nearer."
While working as a biology teacher Welsh would take her pupils on annual field trips to Lochranza on Arran. "I fell in love with it. Arran was like all of Scotland in one place."
It was her write-up of an Arran walk for her regular column in the Western Gazette, a newspaper in Somerset, which lead to Welsh becoming an author. Such was the reader response, the paper published an entire book of Mary's Arran walks in 1989.
"It sold 5000 copies."
To this day, Arran remains a favourite. "Generally, I love the coastal walks, especially the cliff walks in Shetland, which are absolutely beautiful. Orkney is a bit more tricky. It's got many more fences and lots of cattle."
In those early days, Welsh used to walk with her beloved collie Cami. It was when she was working on book number seven that she met Isherwood. "All my illustrators had fallen by the wayside and Chris offered to do the drawings. She's done all the illustrations since then."
So, how do you set about writing a walking guide? "It's quite difficult," says Isherwood. "When we started, you had to work out every walk exactly from the map and by talking to people, but there are a lot more walks signed and waymarked now than there used to be. We go with local recommendations if we can get them."
The pair hire a cottage for six weeks and use it as a base. During the walk, Isherwood will take photographs from which she will later do the illustrations. After a day on the hills, they work until late on the write-up and drawings. "If you are up on a Scottish island and you can only stay there for six weeks, because of the cost, you have got to walk by day and work by night. In the evening Chris is drawing and I'm at the computer and my husband Tom is cooking the cabbage.
"I'd like to point out that I help with the cooking of things other than cabbage" says Isherwood. "That's all I take you for," says Welsh laughing.
Isherwood, also a former biology teacher, often points out flora such as wood sorrel or stitchwart. "You do a certain amount of drawing as a teacher and I've always liked it, although I was always bottom of the art class. If you'd told me I'd end up doing it for a living I'd never have believed you. You can tell I'm a scientist; I like my pictures to look like what they are supposed to look like. It's fantastic to do this, my hobby, as a job."
When you've walked 6000 miles and lived together for weeks on end, the pair must know each other well. How do they find working so closely? "We get on very well," says Isherwood. "Walking is quite a free association, we're not stuck together. We've got our own areas and we aren't criticising what each other does or anything like that. We are both fascinated by wildlife, both being biologists. It's worked really well."
Suddenly, as we cross the sheepfold and head towards the hill summit, a red kite swoops into view. The wildlife is as much a part of the walk for the pair as the route, a fact reflected in the books.
Earlier this year, they completed their final walk, Norman's Law in the Ochils, achieving their "wonderful, magical task". "We danced, we hugged, we sang, we wept with joy," said Welsh. "We thought Norman's Law was the most wonderful hill in Scotland. It was on this summit that we had realised a dream."
In total, there are 21 guides, each describing 40 walks. Even once a book is published, however, the pair can't relax. Welsh is often contacted by readers (or landowners) alerting her to changes to paths or criticisms of the route and they periodically revisit areas and revise editions.
"Any rude letter, we have to sort it out," says Welsh. "They'll say, 'There was no path when we went' or 'The kissing gate had a dead sheep in it. Do you think this means that the farmer doesn't approve of you using it?'
Just then, near the foot of the wood, a willow warbler breaks into song. Welsh is delighted; it's the first time she has heard one. Even at this stage in their walking career, there is always something new to discover.
The Walking Scotland series by Mary Welsh and Christine Isherwood is published by Clan Walk Guides www.walkingscotlandseries.co.uk