Carrie Underwood works out for two hours a day.

The 'Before He Cheats' hitmaker has revealed she always makes time to fit in a work out despite being busy looking after 19-month-old Isaiah.

She said: ''When we are on the road, my day is me doing a show, me being a mom - and those worlds colliding. My shows are intense physically, and I'm running around all over the place and singing at the same time. I need to be able to do that; I need to feel like, 'I got this' ...

''We usually work out for 1.5 to two hours a day. That's warmup and cool down. It really is time for me.''

And the 33-year-old singer can't quite believe how old her son is getting now.

She added to People magazine: ''He's getting faster and bigger. He's running and climbing and jumping off stuff at me!''

Meanwhile, Carrie previously opened up about her simple work out routine.

She said: ''You don't need that much. I like free weights and a Bosu, and you're in business. Maybe a medicine ball. I do lots of squats and lunges, and I like to run, I like to run outside if my knee allows me to. But just body weight stuff, push-ups, pull ups. It doesn't have to be complicated.

''I weigh more than I did when I was at my smallest, but I have a lot more muscle tone. I feel like my bum's a little nice than it used to be.''

And Carrie says having a baby has changed the ''dynamic'' of her relationship with Mike Fisher.

She added: ''Your marriage does change a lot when you have a baby. It's a whole new dynamic. If we could keep having babies like Isaiah, we'd probably have quite a few. I would love to have a girl.

''I feel like everybody expected me to have an album full of 13 mommy-and-me tracks. Like, everything would be sentimental and mushy. I'm still me, and this is just another layer. I don't want to lose myself in being a mom.''