The Regent's Canal is maybe one of the nicest walks you can enjoy in London (or at least, near my place).
My advice: stop in Candem Market and buy food (especially if it's Sunday, there are plenty of food stalls: Chinese, Thai, Italian...) and sit by the canal to eat. Then follow your walk through the Regent's Park, next to zoo and finish in Little Venice.
Enjoy the sun, if you can.