Yes, there is a road/trail on the east side of Lock Katrine, most people take the steamer up to Stronachlachar then cycle back. I have walked the western side of the loch from Stronachlacher back to the ferry through the Belach nam Bo and it was quite easy, if trackless in parts, in spring, but bracken would give trouble later in the year. So yes, in theory you could run a circuit of Loch Katrine. There are plenty of other places to run in the area. The Glen Finglas area has a good circuit you could use, which is boring to walk but would be a good run.