You shouldn't need to actually go through Glasgow to get to Fort Augustus. Alternatives -
M74 to junction 6, M73 north, A80/M80 to Perth, then A9 to Dalwhinnie, A86 to Spean Bridge, then A82 to Fort Augustus.
M74 to junction 6, M73 north, A80/M80 to Stirling, junction 10, A84 to Callander, Lochearnhead and Lix Toll, A85 to Chrianlarich, A82 to Fort William via Glencoe and on to Forth Augustus.
Both routes are busy to start with, but avoid the M8 and Loch Lomondside so are probably quicker in the long run and include some great driving roads.