With the current heat, I am wondering how much impact it is on your performance. Say, if you are planning to do a 12x400 workout at 72" pace, how much you need to slow down if it is 95F? Just want to make sure not to do it too hard while get the worth of the hour spend in the heat (About 50% humidity, so it is not really a factor I would think).
If you can point me to some links that shows how to calculate it, that would be great as well.
From personal experience, heat affects everyone differently. The most important factor in how heat affects you is how used to it you are. Since it's the summer and you're doing a relatively endurance focus workout, I would say leave the watch off the first few and run by feel. And expect that the last few will be much, much harder than you're used to - you probably don't even need to do them if it becomes too uncomfortable.
Remember, this is the summer. It's OK that it's really hot because it doesn't really matter what pace you're running as long as you're building base and improving fitness gradually.
Actually, this is not an endurance type of run, but rather a race pace workout (72" with 2 min. recovery). Will the heat has any major impact on it then?
If you're that worried, head to the road and run it as a fartlek: 72 seconds at 4:48 1600m effort, with 2:00 easy jog.
everyone is different (imagine that).
I am awful in the heat. If it is over 90, it adds about a minute to my 5k PR. A guy on my team only adds about 10 seconds in that heat.
It's never fun to go from 14:20 to 15:20, i'll tell ya that for free.
This is a very common problem with runners of all level here in the states.
Runners here in the us tend to over-analyze every detail of their races and training. Many of you do not race enough because of fear of not being in "top shape". A lot of you think way too much about what effort to put in your work outs, tapering , maintaining high mileage . Many of You guys would panic if you don't see the traditional long run on sunday and would lose yourvmind when someone mentions a day off.
Look it's ok to think about your training and racing. It's good to have structured but don't obsess the minor details. Prs come once in a while. Prs don't happen every time you race. It's ok to race once you're past your peak. Work with your body, enjoy racing and stop thinking so dam much!!
For what it's worth, here is a link to a calculator based on the Daniels book:
http://runworks.com/calculator.html
For a short repeat workout like your 12x400 heat isn't going to have much impact on an individual repeat. If you're allowing "full recovery" your interval between repeats is going to be longer. If you're trying to keep short rest intervals, then you're going to have to slow down to complete the set. Not sure the calculator really applies to either situation.
Personal anecdote: I've dropped my 5k PR significantly this summer. The 2 races that were run in breezy, overcast conditions ended with results of 16:15 and 16:02 respectively. Last night I ran a 5k at 7pm in muggy, melt-your-face-off heat (the high in my area was 98 yesterday)... and I finished in 18:02.
The weather destroyed my breathing, and I was basically jogging after mile 1. I haven't run a 5k that slow since sophomore year in high school (~10 years ago) and my body was completely shot afterwards.
Anyway, I guess I'm trying to say don't be discouraged if your splits are, umm, off. If at all possible, try doing this workout in the evening when it's "cooling off" a bit. Good luck, be safe