Noob - HIIT on an Exercise Bike

First off, if you have any cardiovascular medical conditions, ask your doctor for advice and ignore ours.

Adjusting any of the 4 variables you listed will improve your cardio conditioning, so it really comes down to personal preference.

If you don't do enough walking throughout the day, consider adding 30 minutes of lower-intensity cardio on most of your non-HIIT days, for general health.

A good fitness program should include strength training too, which you can do on your non-HIIT days. New Rules of Lifting for Life is a good program to try.

For weight loss, all that really matters is consuming fewer calories than you burn each day.

Losing weight requires a calorie deficit. If you do not control your intake, what you eat, the exercise will not result in weight loss. In terms of exercise, Interval training, especially HIIT, is not recommended any more than 3 days a week, and for HIIT, even 2 days a week is probably too much for many people. In terms of endurance, while HIIT will help at some level, it won't help for longer term endurance as much as simply doing a steady state cycle on your exercise bike for a longer period of time.

At this point you have an exercise routine that is 26 minutes long, however, 8 minutes of that is warm-up and cool-down. Also, much of it is recovery time, at total of what 12 minutes.

My personal suggestion would be add in some days where you go at a steady rate for 25-30 minutes to get your overall activity time up closer to your goal bases on AHA guidelines.

I, too, would mix maybe a couple days of HIIT with some longer steady-state work for good balance.

The steady state doesn't have to be "just pedal": To make it more entertaining, you can mix up speed work with higher resistance "hill" work at a slower pace, to keep your heart rate in a steady state range but have some variety. Unless there's something weird about your stationary bike, you can also mix in stuff more typical of spin classes, for variety: some standing jogs, some aero position, some hovers (upper body very still, relatively slow cadence, while standing or in a forward off-the-seat position), some pieces where you alternate fairly quickly (2, 4 or 8 count) from seated to standing to forward/standing (we call them "jumps" in spin).

Plug in some music, and use the songs' pace to figure out what to do (slow song = heavier resistance, fast one = speed work, medium = one of the other alternatives).

If you're not sure about intensity levels. and don't have a heart rate monitor (or don't know how to customize it for you), think of doing the steady state work at a level where you could have a conversation, but you can't sing. (Yes, this is over-simplistic.) At the same time, it should be a level you can keep up for quite a while, but with effort.

Kudos to you for wanting to achieve cardiovascular fitness. For now, just mix it up (some HIIT, more steady state), and maybe start learning more about heart rate training if you're interested.

For weight loss, just keep it fun - if it's fun, we want to do it. If we want to do it, we do more of it. If we do more of it, we burn more calories . . . and have more fun. Virtuous cycle!