The Scotland international was sidelined due to ulcerative colitis and the 30-year-old has been pleased with his form since making his comeback.
He said: "I'm getting there. I'm feeling stronger and fitter all the time.
"I don't want this to sound cocky, but I've actually surprised myself in the few games I've played, how well I've done.
"I'm the same as any player who hasn't played for 18 months and missed the last three pre-seasons. I can't wait for this pre-season.
"I'm going to train hard all summer. I have good weights and dietary programmes to get my strength back.
"David Moyes has been great but I feel like I'm starting my career again, like I'm 19, coming into the team, new manager. Every little opportunity, every training session I have to prove what I can do."