If you would like to add more variety and a few more miles to the West Highland Way, there are two very good options. Both would make nice two-week long holidays.
The first could be, add the Clyde Walkway and the Kelvin Walkway to the start of the West Highland Way. This would mean starting at the lovely Falls of Clyde, following the river all the way to join the Kelvin Walkway in Glasgow city centre and on to the start of the West Highland Way in Milngavie. This would add 51 miles and makes the whole walk 145 miles (233km) long.
The Clyde Walkway is a 40 mile (65 km) long, foot and cycle path between Partick in Glasgow and the UNESCO World Heritage site of New Lanark in South Lanarkshire. The path runs close to the River Clyde for most of its length. Unlike other long distance paths it is close to urban centres with easy access to public transport, and is mostly on good surfaced paths.
The Kelvin Walkway is 10.5 miles (17km) between Glasgow to Milngavie. Linking the Clyde Walkway to the West Highland Way. The route follows, the three rivers of the Allander, Kelvin and Clyde. From the centre of Glasgow.
Or the second choice could be, walk the West Highland Way to its finish in Fort William, then follow the Great Glen Way to Inverness. This would be quieter and add about 80 miles (128km), which makes the whole walk 174 miles (280km) long.
The Great Glen Way is about 80 miles (128km) long and runs along the complete lengths of Loch Lochy, Loch Oich and Loch Ness. It also follows the towpath of the Caledonian Canal which links these lochs and creates a through-route from Fort William to Inverness and the Moray Firth. The original route kept to lower levels, but since 2014 there has also been a higher level option between Fort Augustus and Drumnadrochit which offers more dramatic views over Loch Ness.
I did all the above paths in one continuous walk in April 2019, on my Lands End to John O’Groats+ 3 Peaks challenge. They all linked together well and I enjoyed walking them all. I was also able to wild camp every night.
Official West Highland Way website.
By clicking on the above links, you can read about the walks in a lot more detail.