However, he played 45 minutes in Gillingham's 1-0 defeat to Leyton Orient on Tuesday.
He told BBC Radio Kent: "I'm going to work hard at getting as many minutes as possible in pre-season and try to get that fitness back."
King was named player of the season for the 2008/09 campaign but damaged his ankle in a pre-season friendly against Bromley, just weeks after penning a new three-year deal with the club.
Now in the final year of his contract, he said he owed it to the club to perform this season.
"I signed a three-year deal just before the injury so I feel like I have to repay the chairman and the fans. This year is my last and I've just got to give it my best."
It's a real battle for positions. We've got to fight for our place. No-one's taking it for granted.
Simon King
King said that his ankle was dealing well with the stresses and strains of regular football, following a string of appearances in Gillingham's pre-season friendlies.
"[My ankle] is never going to be 100%. I know what I can and can't do now. I've been training hard. I'm very pleased with it.
King is optimistic of the Gills' chances for 2011/12 and is keen to play an integral role in securing promotion for the Kent side.
"I would like to hold down a starting place. As a team I think we've got enough strength and depth in squad that we'll be pushing for that playoff, and especially top three. I think we are good enough," he added.
Gillingham kick off their 2011/12 season at home to Cheltenham Town on Saturday 6 August.
Source: BBC Sport
Source: BBC Sport