From Poolstock (near Wigan) to Middlewich Big Lock along the L & L, Bridgewater, T & M. (discounting Dutton Stop Lock): 43 miles
Lancaster Canal: 42 miles
Union Canal, Scotland - Edinburgh to near Falkirk Wheel: 33 miles [Thanks to Peter King for reminding me of this one.]
London - From Norwood Top Lock to Uxbridge, Slough Arm + Paddington Arm, Regent's Canal (to Camden Locks): about 26 miles
Monmouthshire & Brecon Canal (Wales): about 23 miles
Grand Union from Fenny Stratford to Cosgrove: 11 miles
[Thanks to John Brinnand and "[email protected]" for their contributions.]