Kelly has been out for ten months with a serious knee injury.
"My knee feels perfect and I'm just glad to be back with the team for pre-season," the No.34 explained to the Liverpool Echo.
"I came back a week before the other lads to give me a headstart. It's been tough this week with double sessions but the footballs came out straight away and it's been enjoyable.
"It's been 10 months now so the knee has had more than enough time to heal. Now I just feel unfit, I don't feel like I've had a big injury. I've put it behind me - that was last season, this is a new season.
"Pre-season is about getting my fitness back. I'm looking forward to the Preston game and I'm sure there will be a few nerves leading up to it as I haven't played since last September. I want to get that first game out of the way and then crack on."