If you are considering walking this route yourself, please see my disclaimer. You may also like to see these notes about the maps and GPX files.
I did this walk of about 17 miles (8.5 miles each way, 'guesstimate' only!) on Monday, 15th August, 2011. I had to take my car in to a garage in Milton Keynes for a service and MOT, and chose to go for a 'there and back' walk along the Grand Union Canal while I waited.
I started walking from the garage, which is actually in Kingston on the eastern edge of Milton Keynes, about 8.55am. I first had to follow a couple of main roads and then turn northwards into Milton Keynes village (the original village before the new town was built). I turned left along a bridleway by the ancient parish church, following it all the way to Woolstone - at one point I crossed a bridge over the river Ouzel in a small area of woodland just south of Willen Lake. I passed another old church in Woolstone, and then took a path under a main road that led to the Grand Union Canal (just north of bridge 82, I checked the bridge number so I'd know where to turn off when I came back).
A fairly typical scene of the Milton Keynes road network
Milton Keynes village - I turned left by the bridleway sign in the middle of this photo
The ancient church in Milton Keynes village
The river Ouzel
The old church in Woolstone
Approaching the Grand Union Canal
I followed the canal tow path northwards, with the canal on my left. Beyond the hedge on my right was a hard-surfaced bridleway or cycleway, but I kept to the towpath. Initially I was in a district called Campbell Park, but as I gradually moved northwards I reached Great Linford. I spotted a few wildflowers along the edge of the canal, including Skullcap and Amphibious Bistort, plus two that were new for me - Gypsywort and Trifid Bur-Marigold. After about 2.5 miles the canal turned westwards, though I didn't really notice it as I walked along. After about another mile, still in Great Linford, I reached a short section of canal that I remembered from my Swan's Way walk three years ago.
The Grand Union Canal, just north of where I joined it
Grand Union Canal
Grand Union Canal
Grand Union Canal
Grand Union Canal
Grand Union Canal
Grand Union Canal
Grand Union Canal
Grand Union Canal - this bridge was where I ended Day 1 of my Swan's Way walk and started Day 2.