As Kryten says there are only two branches of the canal you need to worry about - Bulls Bridge and the Slough Arm. Coming out of town Bulls Bridge is a T- junction, left will take you to Brentford, right (and over the bridge) to Birmingham. The Slough Arm junction is easier to navigate - straight on for Birmingham, to get to Slough you have to turn left and cross over the bridge. It is all signposted.
I run and cycle on various bits of the canal quite frequently and know it fairly well out to Tring (having done Tring2Town a couple of times). When I did it Tring2Town started in Tring (surprise !) and finished at Little Venice 45miles later. Other years finished in Brentford and were 40miles.
Berkhampstead is a few miles before Tring and the canal is right next to the railway station, so might make for around 40miles.