Rollers are more work than outside. It's steady so you don't get the little breaks you're used to - coasting here and there or the occasional stop sign or light. Also, because your only momentum is in the rollers and rims, you have to pedal circles more to keep the speed even, and you may not be used to that.

I figure 75% on rollers of what I would do outside comes out about right. I also limit myself to 1.5-1.75 hrs. at a time to prevent burnout, though many people ride centuries on them.

You might also think about easing off on the effort a bit. It's only January.