It's called WORKing out for a reason. Yes, it's "work".
You have to ask yourself, if you can usually do 30 and today only did 5 - why? What really stopped you? Were you physically exhausted? (Look at your past 24 hours - why?) Were you just bored? Were you fed up with doing the same thing over and over? What actually drove you stop?
Working out never gets easier. And it never should. The professional full time athlete and the couch potato on her first walk both push themselves as hard as is reasonable and comfortable to go. The athlete achieves more, and probably lasts longer, but both are pushing, both need to push their boundaries to achieve any improvement, so it "feels hard" for both of them.
It'll never get easier. You just become able to last longer, and go further and faster.
Deb, in New Zealand