I Am A 66 Year Old White Male , Overweight

I am a 66 year old white male , overweight at around 300 lbs. , do not walk enough , have been babysitting my 5 year old and 2 year old grandsons 5 days a week since they were born and am on the computer a lot ( so I do not walk as much as I should ) .

I have had problems with my legs ( mostly the left one ) for a few years now. I had some problems while walking of my ankles and legs seeming to give way and my ankles and foot turning outward to the point that I thought I might fall so I had an exray done a couple of years ago and was told that the lowest disk in my tail bone was worn out ( nothing could be done ) and shown a couple of exersises ( crunch and leg raises ) that have helped that problem .
I have a problem that seems to be getting worse with a feeling in my legs ( mostly my left ) where they feel like the muscles need to be streached . This runs all the way down to the ankles . I also am always tired ( seems to getting worse ) and can fall asleep anytime I sit down. It seems like if I message and streach my leg musles I am not as tired and the feelings in the legs is not as strong .
I am doing the crunches , streaching , and some other exercisies , but the problem seems to be getting worse . I know I do not walk enough .
I also seems to be getting short of breath , but this might be a seperate issue .
I have not made a doctors appointment yet , but I am concerned about possible blood clot or other problems .
Any suggestions ?