I start from june this year, I think it is quiet good time, because based on statistic data, the most people are coming in April/May and July or after September. Normally the people will definitely finish Camino in around 40 days. So make your decision
The trainning, I think better half year before you come. Base on my experience. I have Knee Injury, so I make regular training every week. I was thinking, it is quiet easy for 20-30 KM per day. But, i still get difficulty at the last part of my trip and the way to the seaside. The main problem is, after lang time everyday, your feet feel very tired. Maybe my bag is too heavy, aroud 20-30 KG. my feet muscle get injuries because of long time overuse. It is not so bad, but i feel painful and walk more and more slowly than before.
So my suggests will be, in you planed time, it is very hot in spain. You need in many case take more water and food, if you want to really walk long distance everyday. You need make a plan, just like first week, just walking, and then next week add weight, and after several week reduce weight, and add back. the trainning should be every day, it will help you improve your durability. The content of trianing canbe combination with strength traing, special for your leg muscle, which will help you very much for you over mountains.
last thing, I want to say, is that, you should use this time to find suitable shoes. Because it is less rain in summer, but it rains. If you are just on the time over the mountains, your shoes might be your safety. The route over mountain in many cases, it is very bad and some parts are very percipitous. If your shoes grip are not good, you might slip, it is very dangerous. But on the way to leon, it is very long flat area, your need a very hard downsurface shoe to protect your feet, because there is no real road, stones are everywhere. I take tow paar shoes, one with very good grip and the other one with strong bottom.
you can use pilgrim service through post to sending one bag to another post office, which can hold there for 15 days for you. So you can count your trip and mail your unnecessary things forward and pick up when you arriving there.
good luck.