Canal to Leighton B is a Sustrans cycle path but as usual rough. Have ridden it on narrow tyres but it rattles your fillings out.
The road through from Bletchley via Stoke Hammond is good but can be busy at rush hour, I used to use it to go to work in Leighton. The Hemel road from Leighton is very busy as used as a short cut instead of the M1.
I'd likely go via Stewkley, Wing, Mentmore, Cheddington. I've not ridden the Tring, Berkhampstead, Bourne end part but most of the traffic there would use the A41 dual carriage way.
Rich