Born in British Tanganyika to a white father and black mother, he enlisted in the King's African Rifles in 1957 after finishing school.

During the Uganda–Tanzania War of 1978 and 1979 Walden served as commander of the 207th Brigade in the Tanzania People's Defence Force with the rank of brigadier.

"[2] In 1957 Walden saw a recruitment advertisement for the King's African Rifles (KAR), and, forgoing his original wish to join the navy, decided to enlist at Colito Barracks in Dar es Salaam.

Due to a shortage of administrative staff in the training section, he worked as a clerk for several months before beginning formal military instruction in July.

The following year he joined his unit there, and served as a storekeeper, performed administrative duties, and acted as a translator for the sub-area commander.

In 1960 a cyclone struck the island and Walden was given charge of co-ordinating the distribution of disaster relief supplies with the Red Cross.

In the late 1960s he held various responsibilities, including command of a Tanzania National Service camp in Mafinga, Iringa while holding the rank of major.

[2] During the Uganda–Tanzania War Walden served as commander of the 207th Brigade in the Tanzania People's Defence Force (TPDF) with the rank of brigadier.

[7] Ugandan forces occupied territory in northern Tanzania in late 1978, and during the TPDF's counter-attack Walden and his men recaptured Minziro and a sugar plantation.

The brigade moved single-file through deep water and briefly lost contact with the local TPDF headquarters when its radios were rendered inoperative by the wet conditions.

The 207th Brigade advanced into the western portion of the city,[10] and Walden oversaw the capture of President Idi Amin's residence.

He spent several nights leading wildlife rangers on patrols in the desert, successfully reducing illegal hunting of elephants.