Ahhh, Intervals....my favorite workout! I do them 3 times a week. It is something I have done alot of research on and asked alot of questions about.
Following is a basic breakdown of what interval work involves and how it works:
HIIT workout -
1. Intervals
2. 5 min. slow
3. 20 min. steady pace at or near the same speed as the slow speed of the interval.
There are 3 major steps in of fat metabolism being:
1. Mobilization
2. Transport
3. Oxidation (the actual burning part)
The Intervals (fast and slow combo) =
Mobilization (– mobilizing body fat (triglycerides) breaking it down into free fatty acids)
5 minutes slow =
Transport - gives the body a chance to draw as many of the triglycerides (fat) into the blood stream for burning off
20 minutes steady =
Oxidation - IMPORTANT! if you don’t actually burn off that circulating fat (i.e. - not doing the 20 minutes), you’ll redeposit it!

So try not to skip this portion.
As steady state cardio uses fat as its primary energy source, it will use the circulating fat as it is readily available.So....a good basic starting point for interval work whether it is on a bike, or elliptical, or treadmill, or just walking/jogging outside could be:
2 minutes at a slow speed then 1 minute at a fast pace / repeat this cycle 3 times. Then go slow for 5 minutes, this is when the fat releases into the bloodstream. Last (most important!) 20 minutes of "steady state" pace (roughly the same speed you were doing the slow intervals.)
Another variation is 1.5 minutes - slow then 30 secs. sprint - 3 cycles and then the 5 min. / 20 min. parts.
However many cycles you choose to start with (3 is what was recommended to me) stay with that number for a week before adding another interval.
I have a journal thread in the Workout Journals. It's called "Work in Progress - FitoverForty" You can look at some of the most recent pages to see some examples of the HIIT workouts I've been doing. Not trying to "toot my own horn" just trying to give you some ideas for your own intervals and a little info and/or help, 'cause I sure needed some when I first started!! Hope this helps you! Intervals are the best thing in the world when done right to blast fat and condition the body!
I've been reading your posts and just wanted to say, you are doing great! Keep up the hard work, you have the mindset and determination needed for success, and I've no doubt you will achieve the results you are looking for.

another good link to read about interval work is:
http://www.intervaltraining.net/Interva ... _Loss.html