if training for an ironman how long should these be for? what sort of intensity level?
My suggestion is to throw a tempo run that is significantly faster than your ironman race pace in once a week. Do a warmup and then run tempo for 30 minutes or so in the middle of the run. I also try to add this type of surge into my long runs occassionally. Even though you will be running much more slowly in the ironman it is good to train at the higher level of intensity and not always at ironman pace. Ironman race pace is usually way slower than your regular tempo pace so you don’t want to slow down for your run to match this pace … as an example … I ran 1:30 last Sunday at 7:00 minute/mile pace … My marathon at Kona was 3:15 and I have a hard time running that slow in workouts but no difficulty running that speed in ironman races 
The classic tempo run should last about 20 - 30 minutes, but you need to build up to that over time. Start with 5 to 10 minute chunks of time and add 5 minute sections over time. The intensity or pace should correspond with the type of pace that you could hold for a race that would last one hour. If you race a 10 miler in 60 minutes, then your tempo run pace is roughly 6:00 min/mile. It’s a bit slower than 10K race pace and slightly faster than your 1/2 IM race pace. This is one thing that a HRM can be a real help for, for the inexperienced. If you do know what your various HR limits are, then you can monitor the HRM and surf along the edge of your Lactate Threshold( 160, 165, 150, whatever) which is roughly your tempo run pace. HR takes 5 - 10 minutes to stabilze so this is why the HRM is particularly good for this sort of workout.
You should feel relaxed, but moving fast and efficiently. If going for 20 or so minutes you should not really start to struggle until the final few minutes. If you are hurting and struggling after 5 minutes, you are going to fast!
It’s a great pace to train at. For most/many runners all they really need to do is a few longer runs a week a Tempo run and a hill workout, and if all of these are done right, they should yeild some serious positive results.
IM run pace is basically long run pace (easy-to-steady). You’ll be doing plenty of that, so tempo runs should be much quicker. And yes, tempo runs make your IM run fitness better, even though you won’t be running that fast in the IM.
I do tempo runs at a little faster than open half marathon pace; a little slower than 10k pace. I do them one of two ways.
One is to go 15 min easy/15 min “near tempo”/15 min “full tempo”/15 min easing slowly back down to a last 5 minutes jog.
The other is warmup 10 minutes, and then go 20 minutes full tempo. Walk for 2 minutes, turn around and come back 20 minutes, and push the second 20 a little harder. Cool down jog 10 minutes.
Another important run for marathons and IMs is a marathon-pace run. Only about every three weeks, go for 1:00 to 1:30 at open marathon pace. A great supplement to the steady IM-pace runs and tempo runs.
awesome. thanx guys