In reply to Andy 976853:
When I worked in publishing the letters I dreaded most were those 'I wrote this for my family but everyone who has read it loved it and thinks it ought to be published.....' Take it from me, your family and friends know sod all about what should be published.
That said, there are loads of walkers and dog lovers and as Toad say, there is a market for this kind of thing but I think the key element would be humour. If people pick up a book like this they don't want a guide book or a tale of woe(the bit about mental health rang a bit of alarm bell). They want a bit of light relief to dip into before they nod off or perhaps to keep in the loo. It doesn't need to be full of belly laughs and certainly should not be a string of jokes but just needs to bubble along lightly at a gentle pace and leave readers happier than when they picked it up.
To get an idea there are loads you can look at, not just on walks but bike journeys (Fat Man on a Bike developed into a TV series), boating (classics like Three Men in Boat plus loads of more recent, often canal-based stuff).
Good luck if you decided to go ahead.