31-year-old Walker made 70 appearances in an injury-hit five years with the Robins, but has been unable to play since 2002 because of an injury to the tissue between the base of the thighbone and knee joint.
His contract ended at the end of last season and he had been handed a short-term deal to prove his fitness, however after undergoing another operation Walker has conceded his days at Whaddon Park are at an end.
He said: "It is nearly a year since I last played and I am grateful to the club for the chance they gave me to get fit this season.
His contract ended at the end of last season and he had been handed a short-term deal to prove his fitness, however after undergoing another operation Walker has conceded his days at Whaddon Park are at an end.
He said: "It is nearly a year since I last played and I am grateful to the club for the chance they gave me to get fit this season.
"Unfortunately it hasn't worked out because there were complications. I had to have another operation, but I can continue my rehabilitation at the club."