Don't overlook England's canal network, Beth - towpaths make very pleasant walking.
The options and variations are mind boggling - how incredibly lucky we are to have so many miles of beautiful walking available to us.
I recall a good towpath walk from Birmingham, down the Grand Union Canal, then west on the North Stratford Canal, finally returning on the Worcester and Birmingham Canal, a fine 36 mile loop. ..and you can walk through the Netherton Tunnel!