On rollers, your gears are your resistance setting. Too much resistance, gear down. You want enough resistance so that it's really, really hard to hold 30 mph in top or almost top gear. There are different levels on your set for different rider strengths. I like a resistance that puts me in a steady zone 2 at 15-18 mph for most use. I also like to have enough that in my top gear, I'm in high zone 3 at 50-55 cadence.

You'll notice that resistance on rollers is more sensitive to tire pressure than out on the road. More pressure = less resistance.

You'll find that rollers will tire you more quickly than riding outdoors because there are no descents, stop signs, or traffic lights. That steady effort is the whole idea. I figure it takes about 75% of the time to get the same effect on rollers as it does outdoors. Maybe only 50%.