Don't worry about it unduly. The folks that fail (or hate it) are the ones who try and do it too quickly, and without any preparation. You've got a good route, with stops well spaced out. You've done some preparation, so your feet will be a bit hardened already, and you're not trying to carry all your worldly possessions either. Take it steadily and you'll be fine!
There's not much point in ranking the climbs, as everybody will find different bits difficult. To take each of your sections in turn though:
Milngavie to Drymen - Pretty flat. Am I allowed to say boring?
Drymen to Rowardennan - Small hike over the shoulder of Conic Hill (Balmaha) if you wish,
but can be bypassed by sticking to the main road from Milton of Buchannan to Balmaha, and then the path around the lochside {edited for clarity, and also to say don't do this - see post below}. Otherwise flat (because it runs along the loch)
Rowardennan to Ardlui - Again no major hills because you're following Loch Lomond. However, the path does go up and down a bit depending on whether it's right by the loch, or going up higher along the side of the hill.
Ardlui to Bridge of Orchy (which is the one that scares me!) - Why does that scare you? (is that your long day?) It's a long drag up, but not particularly steep. Start early, put one foot in front of the other and you'll make it.
Bridge of Orchy to Kingshouse - A few minor hills, but really you're up on the high plateau of Rannoch Moor, so nothing too dreadful.
Kingshouse to Kinglochleven - A steep ascent up the Devil's Staircase, then a long descent into KinlochLeven, which is at sea level.
Kinlochleven to Fort William. A steep ascent up and over into the Glen, then down into FortWilliam.
BTW, if you haven't already, add some
tick hooks to your first aid kit, both for you and the dogs.