Wobble to Death by Peter Lovesey (1970) W. Clement Stone 

A six-day race is organized in London with two prominent pedestrian racers and a group of known local racers.  One of the top men is being cuckolded by his own man.  Part way in, there is a death by poisoning.  Some racers take a small amount of strychnine as a tonic, but this man had way too much for it to be of his own application.  Then his trainer dies by way of gas in his tent.  Only he has a bloody dent in the back of his head.  On the last day of the race, it appears the promoter’s assistant has made off with the winnings.  It’s up to Sargent Cribb and Constable Thackeray to catch the thief and solve the murders.