My favorite:

uphill sprints as far as you can at top speed

walk downhill and recover*

repeat untill there is a drop in speed/time

*recovery can be short or longer (but I don't rest longer than 2 minutes myself), depending on what metabolic/energy pathway you would like to adress.

added bonus: after last uphill sprint, rest on top of the hill, and then do lunges on your way down. that will fry your legs for sure

and if you're not ready after this: try sprinting one last time, but backwards :P

I like uphill sprints, and as a physical therapist, I advise them to some of my patients because the impact of uphill running is quite low.

A couple popular interval programs are:

Tabata's and Taku's. You can google each to get the details.

Here are a couple of articles from Mark's Daily Apple that 'll give you a start:

• What are Tabata Sprints?

• Dear Mark: I Hate Running

FWIW: I train in martial arts and we do a LOT of tabata workouts (plus lots of kettlebells, yeah!). I am not good at motivating myself through intense workouts - training in a group is what works for me.

Our warmups before regular class (sparring, weapons training) include lots of tabatas in different forms. So, one thing that's worked for this chick is to find a group of people training in the same way.

In addition to everyone else's great posts, I suggest working on specific distances and functionality in sprinting.

Mix in days of 2x10x10m (2 sets of 10 reps of 10 metres), followed by 5x20m with decent rest in between - or something along those lines. It helps to work on acceleration and short-distance power. Maybe throw in a sidestep or quick turn-around halfway between running the full distance (50m sprint, quick turnaround, 50m sprint).

Once you get fast, sprints and all forms of running becomes a blast. That's good for exercise, and good for just plain life.

I need to chime in about all the exercise doesn't cause weight loss hooey

I beg to differ, exercise doesn't cause acute weight loss, it sure changes your hormonal profile tho, and that IS what causes wright loss. You have to do exercise that bumps IGF, Testosterone, HGH... Some like HIIT does exactly that

I like doing 10min of alternating All out sprints and slow walk till I've caught my breath

Personally the best card hiit I've found is ultimate frisbee, start stop walk run sprint jump, very hunteresqe

Ultimate ftw

I'm always annoyed by links that lead to commercial products for sale. If you have tried PACE and it works for you--fine--but please warn people that they won't learn anything about the program unless they pay for it first before you send them on another wild goose chase.

I find the easiest way to get going with sprints is to engage the body through a warm-up jog of 0.5 - 1.0 mile and then decide to kick it into high gear by sprinting at top speed for a reasonable distance and then to return to a jog for probably double the distance and repeat 10-15x. (For me, this usually works out to running a warm-up of 10-20 NYC blocks, sprinting 1, jogging 2, and so on).

Tabata's are super tough to do mentally (had to be said) and in the context of a full-workout regimen, require long recover times.

Another two methods of sprinting that are a little more fun is to start off lying on your back or stomach and shoot up for a full spring; the other is shuttle runs (more agility than pure sprint).

I just started using a heart rate monitor. I calculated my maximum heart rate and then an optimal zone *the range of which lies between 70 and 80% of my maximum rate). Now when I go for a run I warm up to the lower range limit. Once I hit that number, I sprint until I'm at the maximum of the upper range (80%). I then switch to a comfortable jog until I am again at the low point of the range (70%) at which point I begin again. This enables me to do one mile with effective challenges to my heart instead of three miles of struggling to keep going at an ineffective rate. I used the monitor in the same way, yesterday, on a 14 mile bike ride. At each mile marker I completely rested until my heart rate was in a normal range. Then I took off and got up to the highest speed I could maintain for the conditions of the path and remained there until my heart rate was at 80% maximum. Then I slowed down and stayed at a speed that kept me just above the 70% minimum range until the next mile marker. Surprisingly easy and fun yet I think I got a much better workout than I usually do at a steady cadence.

Don't do Tabata's. Simply put, they may get you out of breath, but you are not going to receive the supposed benefits. HIIT is good for body recomposition although it is also by no means a miracle workout. I used to like a work/rest ratio of 15/45, so 150yd sprints would be a good distance for that.

400m are also good as long as you're using the appropriate rest periods. Just make sure not to overdo it. Remember, diet is the most important factor in body recomp.

This is what I do on a treadmill. 10 minute warmup @ 6.3 mph followed by 1 minute at interval speed, followed by 30 seconds at 3.0 mph, then 20 seconds at 6.3, then 10 seconds to bring it back to interval speed. I repeat this 10 times and I'm done. Total time of workout comes to 30 minutes. I started with an interval speed of 10 mph and now do 10.5 mph. The workout leaves me feeling like I got a good workout, but not exhausted, which was a problem before. I do the workout once or twice a week depending on outside running conditions and how I feel. I'm 53 and prefer this workout to 20 minute tempo runs. If I run outside, it's generally an easy 90 minute (10 miles) run on trails.

This workout is at least as effective as anything I did before and has the benefit of leaving me less beatup so I recover quicker, which is important at 53. I eat paleo (low carb) and usually do this workout after work around 4:30 pm, having eaten only breakfast (eggs, bacon or sausage, milk with heavy cream) around 6 am, that is, after 10-12 hrs of fasting.

Turn your body from a sugar burner to a fat burner through diet (strict paleo) and you won't have to exercise to try to lose weight.

For the fitness aspect, do as Steve suggests and check out the P.A.C.E. program. You will build powerful lungs, build muscle and get fit very quickly.

Dr. Al Sears promotes (pioneered?) an interval workout routine he calls 'PACE'. it's the best fat burning method I've seen and is similar to what geelyn described above.

here is a link to his site: http://www.alsearsmd.com/pace/