12.5 is indeed very low. I'm on 75mg. Of Metoprolol Tartrate. The nice thing about the two is the doseages are the same. The difference of course is that I must take mine twice a day.
It's hard to believe that dose would be responsible for your weight gain, especially with your activities. I found that when I hit my 50's. I had to be very, very careful to maintain my weight. I now eat a Paleo diet, and shun carbohydrates as much as possible.
I started on 25mg Metoprolol succinate about 35 days ago. I have put on some weight but for nearly 3 weeks have not worked out much. The first week I was just getting used to it. I was winded from swimming 1/6 my usual distance! Then for two weeks I was on business travel and eating, not working out. Then last week I swam 12 miles in six workouts. And now this week I already swam 6 miles. I feel I started to get more room in my jeans.
The dose you take is half of mine. I'm in my 50s and have been athletic all my adult life so I forced myself to get back into the workout mode as soon as I could. My cardiologist endorses my swimming.
Metoprolol as well as other BB suppresses the heart rate, so I'm not surprised that you feel winded upon initiating any physical exertion. You can work through this though if you persist. I start my warm-up long before others do, in order to get my heart up to "working speed". For me, that usually means some fast walking into a light jog into some brief sprints before I ever put on my skates. There's a fine line between warming up and over working yourself before you actually set out to do what was originally intended. You just have to find it.
I found that if I tried to just go out and push it, I got this cinder block on the chest feeling. My heart just wasn't responding to the demand as quickly as I wanted. At our age though, what you want, and what you get are usually different.
So push through that initial fatigue, and I think you'll be fine once you're on the other side.