treadmill exercise

I started treadmill and weight lifting two weeks ago, 30 mins at 5..5
and incline 2.0. This Monday, I decided to do 2 sets of 30 minutes runs
with a break in between doing weights. It was easy, the 2nd 30 minutes
run was a cinch. The break was about 1 hour and 30 minutes.

Today, I wanted to repeat what I did yesterday, except the break was
shorter, around 45 minutes. But, I didn't not finish the 2nd 30 minutes
run because it was way too hard, my entire body just didn't feel right
and my muscles were like stones.

I would like to know if it was because of the fatigue I had from Monday
or, perhaps, I simply need to take a longer break between the runs. It
was such a huge difference that I didn't believe I made the 2nd run on
Monday. BTW, I exercise every weekdays for the past two weeks and take
a break on Saturdays and Sundays. I'm only 23, and I think any fatigue
would go away after a long night of sleep, but that's just what I think
anyways.