Can you walk the dog during the day with your child?
When I was a SAHM I would go out, after breakfast, with my toddler in the pushchair and the dog on the lead and we would all go out together for a walk and some fresh air. We would head to a field near the house and I could get my DD out the buggy to walk a bit if she wanted, run on the grass, pick daisies, play games. The dog could be let off lead to run after the ball, which I go my DD involved in and she would run around with the dog.
Then at about 4pm, we would do it again. We even did the walks in the rain, as I had a rain cover for the buggy.
That way, dog has already had walks/exercise during the day and then when DH comes home and does bath time, that is when you get some time to yourself to do what you want to instead of walking the dog at that point.
Just a suggestion, but I was lucky in that my kids loved to be in their buggy and would enjoy the walk and fresh air as much as I did.
Meet a friend, attend a class, go to the gym for an hour, during that time. Or just relax and read for an hour.
Things like meeting friends, I found, need to be planned in advance. So think about when you might be free and message them to ask if they can meet up, if not, then when? Meeting friends doesn't just happen, you need to be organised and pro-active.
I attended lots of baby and toddler groups when my DD's were young and grew a nice friendship group from those. We would go for coffee after, or lunch, and we went on nights out once in a while. But we were each others sounding board and support and it was great to talk to other mums and have some adult company.
The time your Mum has your child, do something with that time for you. Go to the gym, wash and dry your hair, read, meet a friend. Go for a mooch round the shops and grab a coffee. You are very lucky you have that time so make sure you use it wisely.