The first day of the challenge should feel like it needs a bit of effort, but not too much. If you really struggle with day 1, then you probably won't get through the challenge. The idea with the challenges on this site is that the first few days get you used to the exercises and the daily habit. Usually about halfway through the challenge the effort levels start to increase more quickly.
Avoid challenges that increase effort levels very quickly - they're simply not realistic. It's unlikely that, for example, you would be able to increase how many squats you are able to do by 10 a day for 30 days, or increase your plank hold time by 10 seconds a day for 30 days.