This years race is on the 22nd - 25th April
The Devizes to Westminster International Canoe Race is 125 miles long and has 77 portages.
It starts in Devizes, Wiltshire and finishes opposite the Houses of Parliament near Westminster Bridge.
The race is during the Easter weekend and has been challenging canoeists and kayakers since 1948. 
 
The route goes along the Kennet and Avon Canal for 52 followed by 55 miles are on the River Thames and the final part of the race is on the tidal portion of the River Thames. 
There is a gruelling non-stop version of the race and it is in fact the longest non-stop canoe race in the world. 

You can find more info on it here