The 24-year-old played in Saturday's 3-1 victory over Dundee United after first-choice goalkeeper Cammy Bell suffered a broken wrist, and will be in goal when Killie host Stenhousemuir in the Scottish Communities League second round tomorrow night.
Letheren waited almost two years for his Killie debut, playing two games towards the end of last season, and he is looking to seize an extended opportunity.
The Welshman, whose only previous first-team experience was a substitute appearance for Barnsley, said: "I didn't wish that injury upon Cammy but that happens in football and I've got eight weeks to go out there and prove myself.
"I'm looking forward to it and going to grasp it with both hands.
"I'm not a nervous boy, I'm a very confident boy. I've played three SPL games now and been on the winning side three times and I'm going to do everything in my power to keep it that way.
"Obviously I'm going to get better the more games I play. I'm going to work as hard as I can and see where we are at the end of the eight weeks."
Letheren added: "I know I've got to play out of my skin to keep it because Cammy is going to come back in, he is a Scottish international.
"Basically it's mine for now. If he does come back in then as long as I do myself justice for later on in my career, then I've got to look at it that way.
"But at the moment I'm just concentrating on these eight to 10 weeks and showing people what I can do, and helping Kilmarnock up the league and through in the cup."
Letheren feels his chance is a reward for persistence after seeing himself down to third choice when Finland international Anssi Jaakkola arrived at Rugby Park.
"I came in and it was just me and Cammy and Mixu Paatelainen decided to bring Anssi in and he took my place for a little while," he said.
"Obviously I wasn't happy with that but I just had to accept it. I had to dig in, I could have just packed it in and gone elsewhere but I chose to dig my feet in and train hard with the keeper coach, Billy Thomson, who has always backed me to play in the SPL.
"I dug in and this pre-season I got the number two spot and I've got this chance now and it's down to me."
Killie begin the defence of their trophy against Martyn Corrigan's Stenhousemuir side.
"We can't go in with any complacency, we have to treat them exactly like we did Dundee United," Letheren said.
"We have been told about their strengths and weaknesses, we know they are undefeated this year."