Walker first competed for Gibraltar at the 2000 IAAF World Cross Country Championships in Vilamoura, Portugal where he placed 158th in the senior men's race in a time of 46:48 and finished 149th in the men's short race in a time of 14:15. The following year at the 2001 Island Games in the Isle of Man, Walker finished 4th in the men's individual triathlon with a time 2:06:30. At the same meet, Walker also recorded a personal best and national record time of 10:34.95 in the final of the 3000 m steeplechase. In July 2002, Walker made his debut at the Commonwealth Games by competing in the men's triathlon at the 2002 Manchester Games. He finished the course in 2:06:15.19 and placed 26th. Three months later, Walker competed in the 2002 Ironman World Championship in Hawaii. He finished 160th out of 280 in 35–39 age division and 767th out of 1605 overall with a time of 11:22:55. At the 2003 Island Games in Guernsey, Walker won the silver in men's individual triathlon and alongside Harvey Gurry and Anthony Perera finished 6th in the team event.