Most of my exercise routine is cardio on a Nautilus stationary bike. I've gone from no exercise to to build up to 60 minutes in a single nightly session, six days a week. The intensity levels vary in a "rolling hill" scenario where, after ~10 minutes of warming up, my heart rate ranges between 70% to 85% of the maximum…
I found this blog post very interesting: http://jamesclear.com/master-one-thing