These spring to mind
1) There is a loop from the visitors center at Argaten down the peninsula over to Lochgoilhead. Bit of a pull up the tarmac road to the top of the rest and be thankful though… Easy enough to spot on an OS map.
2) Another one from arrochar up to loch sloy dam (i think it that one anyway) then back some fairly enjoyable single track.
You could hop on the ferry over to inversnaid if you want to do a bit of the west highland way. I wouldn’t do that section myself, but plenty seem to do it…
There is another route near Furance at Inverary that takes you over the hill down to loch awe, I think that now has an offical name but can’t remember it. Not particulary near loch lomond, but in the right direction.
DH