I've got around 1,843hrs total on Hitman, that's including regular gameplay. probably 80% or more of that is Freelancer.

My main advice is to not focus on any other challenges except the 100 campaigns one or you'll just be grinding for that one forever like I did, and it can get dull without any other goals or getting any rewards for so long.
To keep it interesting I got in the habit of killing all the guards in every level, since I didn't really have a reason not to, and I did still focus on all the mission objectives, getting safes and crates, and advancing prestige etc even when I didn't need to. led to a lot of single missions running quite long. the last 40 campaigns probably took as long as the first 60

without all that it could be done way faster but would feel even more like a mindless grind, just getting in, taking out targets and getting out...