The Queensland Reds and Wallabies lock has spent the last 308 days on the sidelines due to a succession of injuries.
Horwill was first rocked by a hamstring tear and his return to action, scheduled for the beginning of the Super Rugby season, was derailed by an ankle injury.
With the arrival of Warren Gatland's Lions Down Under looming on the horizon, Horwill is thrilled to get back on the pitch against the Bulls tomorrow but he admits the coming hours will be the toughest.
"There's been excitement, anxiousness," said Horwill. "This is probably the hardest part: sitting around waiting and managing the emotion. I will probably sit around and try and focus on other things other than rugby.
"I thought about how much I have missed out on I counted 308 days between football games.
"That's a fair amount of time not playing when you are a professional footballer. I am looking forward to getting back into the swing of things - this is the final hurdle."
Horwill is one of five changes by the Reds and this is the beginning of his preparations to face the Lions.
The Wallabies are sure to be faced with a bruising physical challenge from Gatland's men but Horwill reckons it doesn't get any tougher than the Bulls.
"Physically, it is hard to find a tougher team," added Horwill.
"They have made a number of changes but they have swapped like for like; they are not lacking for size."