Johnson made only 13 appearances last season after struggling with shoulder and groin problems.
The former Everton and Crystal Palace striker is determined to get back to fitness and he is hopeful of returning to action in the next few weeks.
"I've done everything I can to make sure I haven't lost any speed," Johnson told the Daily Mirror.
"I've worked on my glutes, my hamstrings and my core strength - everything that gives me pace. Of course, we'll see when I get out there, but I'm confident I can be as fast as I've ever been.
"If it's the end of September, October, or whenever, that's when it is, but I can see the light now."
Fulham boss Mark Hughes has confirmed Johnson is making good progress with his fitness and should be available for selection in October.
"Andy should be back in four weeks," Hughes told the Fulham Chronicle.
Johnson's return will come as a massive boost for Hughes with Bobby Zamora ruled out until next year after breaking his leg against Wolves on Saturday.