Ah! It's not a route anyone would expect, but in my marathon-running youth I gave it serious thought after studying the maps, if only to be able to say "yes, I ran across Scotland one afternoon". Unfortunately, thought was all I gave it.
Anyway, Billloudon, here we are. Consult tide tables. Start on bank of Clyde in centre of Glasgow. Dip toe in water at high tide. Follow the north side of the river then the Firth and Clyde Canal until it joins the River Carron just outside Grangemouth. Follow river half a mile and dip toe in Forth estuary.
The canal starts well downstream at Bowling Harbour, but the upper tidal limit in Glasgow is the Albert Bridge, just east of the centre. You would need to go north through the streets for half a mile to pick up the canal.
More seriously, this would be a cracking run. On a very well maintained towpath, through lots of countryside and industrial heritage and it even follows the Second Century Antonine Wall for most of its length. About 37 miles, although I haven't measured it.
D'you know, I wonder if I could still do it....
