So I've managed to start "exercising" now that I've got some help with my toddler. I used to walk 1-2 hours a day carrying my daughter, but can't do it anymore for over an hour now that she weighs over 20 lbs. Lower back hurts. I started doing some squats, jumping jacks, wall-pushups, and try to do pull-ups a few times a week. But I think the intensity is lacking. I always stop once I feel my heart start racing. I have never lifted weights or played sports so my body is not used to any sort of intense workout. I'd like to change that. Right now I do yoga a few days a week, do some pushups/jumping jacks/pullups a few times a week, elliptical for an hour (but low intensity) a few times a week. How can I improve my fitness? How can I strengthen my lower back?