The former Fulham forward has seen his first-team chances limited so far this season and has struggled with various injuries all campaign.
However, the 33-year-old insists he is fit and ready to get back amongst the goals.
"I had a little bit of a hamstring injury but that was 12, 13 weeks ago," Johnson told the Fulham and Hammersmith Chronicle.
"I've been fit now for a while now.
"I'm quick and sharp and working hard in training, working on my strength work, hamstrings, glutes and quads and that's something I do daily."
Johnson has been linked with a move back to the Premier League in January, but insists he is more than happy to remain at Loftus Road.
"I'd love to stay," Johnson added.
"I love it here but it's not down to me. Hopefully, towards the end of the season and fingers crossed if I've done enough, I will get a new deal.
"If not I'll have to look at other options."