Fortune, who had surgery after rupturing his Achilles tendon against Crystal Palace in late January while playing for Charlton, signed a short-term deal at Bramall Lane last month and after recovering full fitness is ready to challenge for a place in Kevin Blackwell's starting line-up.
The 29-year-old said: "I need to play matches to get that sharpness back into my game, the international break gives me time to catch up and I'm certainly getting closer and closer everyday.
"I'm looking to crack on fitness-wise and get out there playing. I've been available for the last few matches, but after this break I'll really be pushing.
"It's been a frustrating year or so through injuries and then minor setbacks have hampered my chances here, but I can see the light at the end of the tunnel now."
Fortune added: "Morgs (Chris Morgan) and Killa (Matt Kilgallon) are two of the best defenders in this division and have been for a while now, but hopefully that will bring out the best in me too."