Yet another PF thread.....?

It could be a lot of things. Depending on where under the arch you're feeling it, it could mabe be posterior tibialis as well. For whatever it is, you're probably good to cut back volume a bit, stretch, ice (heat might also help, but it depends on what it is -- try ice and heat, and use whichever seems to cause the most pain relief). Also massage it. You can use The Stick by standing on it and rolling your foot over it (just use the other foot to hold it in place), or roll the foot on a tennis ball, or just get in there and massage it.

Also give your calves some attention, if it's PF or PT, your calves could be contributing.

Also, look into those new shoes.

Good luck .

From a massage therapist... (and PF survivor!)

First, do see an orthopedic or sports doc to rule out tears or other injuries and to get specific treatment if it is PF.  It's certainly possible it's plantar fasciitis, especially if you've been doing speed work during this intense week, or anything that would make your calves tighter or work them more than usual.  You're right, the plantar fascia runs along the bottom of the foot and is connected indirectly (via your heel) to the achilles tendon and gastrocnemius (calf muscle).

Until you can see a doc, thre are a few things you can try that shouldn't do any harm.  Of course, rest is the first  thing likely to be recommended, but I understand your position on that one!  Maybe backing off on speed and/or strength work would help, though, as well as shifting some or all of your training time from running to cycling and swimming. 

I agree with the previous post that massage, ice, and heat may be helpful.  I would be very conservative with massage and heat on the area that hurts right now, though, as this could actually irritate it further.  Even aggressive calf stretches - often recommended for PF - could cause irritation.  I definitely suggest massage (squeezing and kneading) and heat on your calf and achilles tendon - really try to loosen them up this way.  Also, contraction of the muscles on the front of your legs will cause the calves to relax.  I personally think this one is very important and usually overlooked!  Lifting on your bike pedals instead of pushing can accomplish this.  Do make sure you're stretching your hip flexors (your quad stretches probably get them, too).  The object here is to loosen the parts of the system that contribute to tightening the calf/achilles/plantar fascia or constricting your gait (early toe-off, heel lift, etc) without unduly aggravating anything that hurts!

Icing the painful area a little after workouts may be helpful.  New running shoes are always a good idea (I'm the worst offender of this one!), as is limiting the time you spend walking around in running shoes.  Remember to spend some time in strength workouts on the muscles opposite the ones we overuse in tri!  And do see the doc.

Good luck!  Let us know how it goes.  Where is your HIM?