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What's 177 miles long, has the sea at either end and is celebrating its fortieth birthday this year?

It's Offa's Dyke, the ancient border which at one time separated Wales and England. I spent the day walking along parts of the pathway, which is one of Britain's National Trails.

I think it's fair to say that other paths in Wales (like the Pembrokeshire Coastal Path) may be better known, but the Offa's Dyke trail does attract thousands of walkers every year, including 3,000 determined people who complete the entire route.

I met Rob Dingle, the Offa's Dyke Trail Officer at the standing stones above Hay on Wye, surrounded by thick fog.

A foggy view point

Rob assured me the view was stunning but I had to take his word for it because I couldn't see my hand in front of my face!

It's part of Rob's job to walk the entire trail every year and he explained that as part of the anniversary celebrations this year, a new tourism project has been launched - 'Walking With Offa' aiming to promote the path as a tourist destination.

The trail begins in Chepstow on the edge of the Wye Valley Area of Outstanding Natural Beauty (ANOB) - one of five in Wales and ends in Prestatyn near another ANOB, the Clwydian Range.

The idea is to encourage businesses to 'welcome walkers' along the route, by improving facilities like accommodation and food outlets.

There are also plans to improve public transport links, to get walkers to the area without relying too much on their cars and create more circular walks, realising that not everyone wants to walk 177 miles in one go.

Offa's Dyke trail

But the biggest challenge is the fact that the pathway passes back and forth between Wales and England 27 times along its route, which means that two countries and a lot of local authorities have to come together to co-ordinate the project.

Imagine what King Offa himself would have made of that! His feat was to construct the entire border back in the 8th Century - probably to keep feisty Celts out of his kingdom, Mercia.

Last year around 100,000 walkers completed circular walks that included parts of the Offa's Dyke trail, generating an estimated £2m for local economies. Offa himself would have been horrified.