As a fellow "under tall" person...I am not overweight...I am under tall..." Allow me to offer these reflections and suggestions. I have done six Caminos, and have been "fat" for all of them.
1. 25-30 km daly is unrealistic when you are starting out carrying the equivalent of two extra, loaded backpacks (the extra 50 pounds you mentioned). I recommend you seriously consider adjusting this, at least in the beginning, to less than 20 km. Do not be put off by the "Brierley
Simply adjust your daily starting place to somewhere in between. You may find getting a bed easier as well. If you gain capability as the days and weeks progress, then increase your daily targets. As others have said, you do your own Camino. It is NOT a race. Be the tortoise, not the hare.
2. Hydrate, hydrate, hydrate!!! Those of us carrying extra weight sweat more and need more replacement water, as well as other foods and liquids that contribute to maintaining electrolyte levels (oranges, bananas, etc.). Do not overdue it with beer and wine. It is VERY easy to do in Spain.
Force yourself to consume water on a regular schedule even if you do not feel thirsty. I took three "dirt naps" (sudden onset syncope [fainting]) on my way from Porto to Santiago in April 2015. The doctors in the US later determined that I was likely suffering from heat exhaustion and dehydration.
3. If you know you will have joint pain after walking for a few hours, take an appropriate NSAID BEFORE you start in the morning. Most of these pain relievers take an hour or so to metabolize. This way, the relief will be there when you need it. It works for me.
4. Take better care of your feet than you might do at home. Many folks use Vaseline or something similar to lubricate their feet, before booting up each morning to reduce friction. But, remember that EVERYONE'S feet are different. What works for me (and it does) may not work for you.
5. If you find you need something for your feet, you can pick it up as you progress in most any farmacia in Spain. On my last Camino - I just returned Sunday - I even found Dr. Scholl's gel insoles in a farmacia. Many pharmacists speak at least a little English, and along the Camino, they have seen every imaginable illness a pilgrim can get.
6. If you planned to do 20 or 25 km and you just cannot finish the last 5 - 8 km for the day, there is no crime or sin in stopping at a cafe and asking them to call a taxi to take you to the next stop. It costs about €1 per km. Do not tip, but do round up.
7. If you are going to be on Camino for a month or more, try to build in a rest day-off every week to 10 days. If you arrange these at places like Logrono, Burgos, Leon, Astorga, etc. you will be able to rest up for an extra day. If you are in an albergue, simply tell them you are having health issues and ask to stay a second night. It is not odd, and most albergues will oblige you.
I could go on, but these tips will help you manage your Camino in a sane manner.
Hope this helps.