Sorry this is epic - I just would like a chat about my dogs welfare 
My dog (old bull mastiff) seems sad all the time and I always have this guilt concerning her. She is an old rescue and I guess she has always seemed a bit sad. Unfortunately we all live in a small flat, me, my dp and 9mo ds, its above a pub and she gets to go down to the pub in the late evening before her last walk.
Every 6 weeks or so my dog gets to have a few weeks 'holiday' at my parents house, which she adores - she seems happy there! She has several acres of garden and a large house to wander around offlead and her own sofa there.
Anyway when she is back at the flat, we take her out for 5 shortish walks a day - they are between 10 and 40 mins long - she always gets one 40 min walk a day, the rest are shorter. Her back legs are weak (she has painkillers) and she cant really handle longer - very slow walker. We cannot let her off the lead as she is very fear aggressive towards other dogs. She now sits and waits (very alert and tense) for a treat whereas before she used to go beserk on the end of the lead, so i think we have done well there. Basically she cannot be let off the lead where other dogs may be, so we walk her round houses and small quiet parks.
Whenever I pass her I always say hello and try and stroke her, she gets cuddles but she turns her head away sometimes, so I leave it, sometimes she puts her head in my lap with a sigh - I just cant work her out!
She gets fed 2 cups of dry food a day - I put half of this in a Kong or scatter/hide the biscuits around the flat so she has to sniff them out, this gives her something to stimulate her brain a bit when she is indoors. she is food obseesed, if I play with her, she jumps up and goes to kitchen and expects food? she puts on weight easily so i have to watch her on this.
I am a bit unsure about signals - Ive read that panting, yawning and licking of chops means that a dog is uncomfortable, but she play bows and yawns when it is time for her to go for a walk, also when we look at each other we exchange a 'slow eye close' and a 'small lick of the lips' to each other 
and this has always been a sort of 'are you alright?' kind of exchange - have i been getting that wrong?
She doesnt love the baby, not aggressive but she moves away, which is fine, I always position myself between the two of them, if he is playing nearby and I go and stroke her and reassure her so she has a positve association. If I'm holding him, she will give a small lick on his toes sometimes. I wont let him touch or go near her until he is old enough to know how to be around dogs - it is hard in a small flat but she needs her space.
I know she hates being cooped up in the flat, but so do I! I always have this feeling that Ive let her down, I've had dogs all my life and i know a sad dog when I see one. We got her when she was 5 or 6 about 4 years ago, so she is old and I have no idea about her previous history. She is a lovely dog, she is rarely left alone.
Is this enough? Am I doing enough to keep her happy? Why does she seem so sad?
Thanks for reading this far.