I run 5-6 times a week, usually adding up to about 30-40 miles per week, including track/trail and road sessions.
Generally speaking all my cardio is outside, because a) I like getting outside and b) running on treadmills bores me... although I like being able to listen to music while running on the treadmill. I do try to fit in a couple of swims each week (1 mile) which is certainly inside (woah no outside swimming for me in frigid lakes or bogs
). Swimming also constitutes my nod in the direction of strength training. I have cycles of thinking I should do strength training every now and then where I enthusiastically do 3 sessions of it a week, but it never lasts for than a week or two, as I rapidly get bored by it. Leading me on to:
I do cardio because I enjoy it - absolutely love getting outside and running, especially running cross open country on a summers afternoon! Also love the feel of battling against the elements on a lousy winters evening.
Training up for some fell/road races over the spring/summer.