Harrison is working his way back from a nasty bout of tonsillitis that sidelined the former third-round selection for over a week.
"I think I lost like over 20 pounds," Harrison said. "I had to get a lot of IV's in, so I lost a lot of fluid. I couldn't talk for a little bit. [The tonsillitis] did a lot. I know I missed 10 days, if not more than that, so I've been gone for a while now. Like a long time.
"My body has to get used to like going through [drills] and stuff, so at first when I started out, my body was really tired. But we're going to try and make a step each day. It was tough at first today, but you have to get past the mental part. Hopefully tomorrow I can come out and get more reps."
The defensive tackle caused a stir last summer by reporting to training camp overweight due to personal problems, which prompted the Bears to initially place him on the non-football injury list. Harrison quickly rejoined the team, but made a marginal impact in his second season, recording only one sack and one tackle for a loss.
"Before I got sick, the offseason was going great," Harrison said. "I just got to get back used to it. I've been out for a long time, so man, my body just has to get used to it. I lost a lot of weight and stuff, so I just have to get it back.
"I know Rod [Marinelli] is going to pick it up and make sure I get back to where I need to be. I'm not really concerned about that. I want to get in there and do it. I need to get in there and do it. But coach Marinelli is going to have me ready."
Harrison is ready to make an impact in his third season.
"I'm excited, because I sort of look at this third year like my junior year in college [at Arkansas] when I made a big impact, " Harrison said. "You just go through your stages, like being a rookie is the same as being a freshman or sophomore.
"Before the illness, I was feeling good. I'm still playing both [tackle] positions at the nose and three-technique, so I'm just learning, and this time, I'm actually understanding what's going on as far the game. I'm real excited."
Jeff Dickerson covers the Bears for ESPNChicago.com and ESPN 1000.