Tired Heavy Legs, Weak arms. Any ideas? Desperate for help!

I was walking 2k every day and then back in 2014 I experienced the same thing and within two weeks I could barely walk two blocks. Took two years but it turned out to be myasthenia gravis which I am hopefully getting close to finally pushing into remission. What you have could be many things, (there may be hundreds). I would suggest you start with the simple things, blood tests like thyroid panel, iron panel, vitamin D, complete metabolic panel, hormone panels, the basic wellness stuff. Also ESR and CK for inflammation and muscle breakdown. Statins (cholesterol drugs) can cause these problems also if you are on them. If they don't find something there then it will be CT's and MRI's.

I hope it is something simple like a thyroid issue, a couple of pills and off you go.

ESR is a test that measures inflammation, CK measures muscle breakdown. Myasthenia gravis or MG is typically handled by a Neurologist and you may have to consult one if you haven't already. MG can be diagnosed using a blood test or more simply by giving you the drug Mestinon and seeing if it relieves the symptoms (if you take Mestinon you will have to take a drug to counteract the adverse effects, I use 1mg Glycopyrrolate and Imodium).  

However, my neurologist used a flow chart on which there were about 100 diseases, you start at the top and work your way down and MG was somewhere near the top. And they were a little suspicious because my eyelids were drooping (an MG symptom). He said that about 40% of the people got to the bottom without a diagnosis but even these people could be treated.

A piece of advise if you should wander into this crazy autoimmune world. You need a plan and the goal of the plan should be remission, not stabilization or making you comfortable. Even if you don't make it, that needs to be the goal and you need to get as close as possible. On diagnosis my Dr. put me on Cellcept and Mestinon and when the MG flared we used IVIG, and presently I'm using the Mestinon sporadically. Also my clinic avoids prednisone at all costs.

I didn't notice it for about 1.5 years after the legs went, when I started to see my eyelids, they were blocking my side vision.

As to the MG I think it started in 2009, mostly fatigue, everybody thought it was thyroid. It apparently affected my lungs and respiratory system. My legs went in 2014 and I finally had to retire so I could travel to Seattle where they have big hospitals and clinics to get diagnosed in mid 2016. The whole thing was a very complicated process with many dead ends and much frustration until I found the right doctor. He got it on the first visit.

You might like to try reading "Could my problems be due to McArdle Disease" in the McArdle's section of the AGSD-UK web site. Your brief description sounds very typical. Although it is normally apparent that there is something wrong from childhood, sometimes people do not realise until it suddenly starts getting worse, often after an enforced layoff from activity.