Hi Sarah,
I am sure that the effect of the PM, on your mood, will give you a different outlook on life. I wish you all the best, in that respect.
As to excersise, I know you have the back problems, which limit you, but have you considered getting a dog. I have 2 dogs and they are crucial to my sense of wellbeing. From April to September, I work 16 hour days, 7 days a week, with hadly a break; except the 1to 1 1/2 hours I try and take, for a vigerous walk with the dogs. If for any reason, I cannot take them out, for a few days, my weight goes up. Further, I found the company and unqusetioning loyalty, of my dogs, a huge comfort, in the early stages, of my transition. To me they were mummy's boy ang girl, long before I confided, in any human. In that way, they helped me view myself as female. If you live on your own, the fact, that you have a sentient being, around, who you have to look after, is very important to your sense of wellbeing. Further, if you are out with a dog, other dog owners and dog lovers, will talk to you and you will feel less isolated. In the end, the ststistics say it all; dog owners, live longer.
If you worry about bending down to take hold of a dog, or to put it's lead on, consider a retired, racing, Greyhound, they are the gentlest and most easy going dog imaginable and the collar is easily held, without bending down.