It sounds like your legs are just worn out from so much concentrated training. Although most of it can be considered cross training, it is still a lot that you have been doing in a short period of time. I can't say that I have tried to attempt any ironmans or triathalons but I have been running about as long as you (3 years), run as many 5k's and 10k's in the past year, and I am about the same age. One thing I noted is that I have to give my self some time to recover (at least a day) or else my performance drops. Not to rain on your parade but I had tightness in my calf before and sometime later I tore my achillies. You might want to get a doctor to take a look at it just in case it is a case of achillies tendonitis. Since my recovery, I have been running 2 - 4 half marathons a year but I am still careful of any type of tightness in my calf muscles. You are probably ok, but with the goals you have set, it is best to play it safe.