Monthly challenges are based off you previous workout and exercise data,they are different for everybody.And yes they do get harder as time goes on,this is to get you to work harder and get fitter.
Some challenges are nearly impossible to reach.Whatever goals you set in the activity app,just stick with these and don't go way over your goal or else that over the top figure will become your next monthly figure and it will be a lot higher and harder to achieve.
What I do is once I've reach my exercise goals I take the watch off,so far it's worked because I haven't had a monthly challenge that I couldn't complete for a very long time and I've been doing this for about 5 years now.