I really feel for you Op, I am nearly mid-fifties and although I wont retire till at least 60 (not teaching but a profession) I am already realising I will need to be proactive to build up a social life when the time arrives. Whilst not clinically depressed, I am very low at present after losing my mum, my DS about to go uni, DD having some educational issues. I am struggling at times, and can see that if I was retired, I would find it even harder.
My DH may retire with me, but he loves work much more than I do. So likely to work longer. DC will be early 20s when I am 60, so likely to be independant but unlikely to yet have DC of their own.
Having lost my mum 6 months ago and never having been close to DB (and had lots of nastiness from him) my only contact with him going forward will be an occasional text, if that! I have 2 female cousins I am in contact with but they live 70miles away and already in their 60s with grandchildren.
As for friends, I can appreciate why you havent got any - all the local friends I have built up over years have been other mums, but as DC now older, we have drifted apart. Even the one I have known the longest, was quite close to, we both have dogs, has contacted me only once since my mum died. I know she walks her dog with another of the mums, if I ask I will be invited but it is always me asking!! Plus they all much younger than me, so wont retire at same time.
I have another friend I met through a forum, she is late fifties and retired, we get on well, but she spends a lot of time away either visiting DC in London or keeping her DH company with work. We are trying to sort a first meet-up after summer, earliest she can do is October!! Most local acquaintances have built up a friendship group via kids sport activities or through ladies circle etc. Latter not my cup of tea, plus am exhausted mid-week with long commute, former hasn't been applicable to us.
Similarly with work - I work 35 miles away in a male dominated environment - I do have some female friends and we occasionally meet outside work, but they live miles away in opposite direction and are younger with young children too.
There are some great ideas on here, some of which I will take away. I would also suggest a dog, I have always enjoyed walking and happy to go alone, but having the dog makes me go more when I am not at work. I would love to join a walking club but not possible at present, as cant do weds or sunday walks. But that is likely to be one of our first priorities when DD more independent and doesn't need us at weekends.
I also second some volunteering or studying, plus U3A. Our neighbour is in latter and is always out and about, on trips, holidays etc, I will certainly join when retired! I did OU years ago for work and am now contemplating a general course just for interest now DC growing up.
Hope you can build up some interests, as one pp said, try doing it slowly, one at a time, and in no time at all you will hopefully have a busy life. Whilst losing your job in the way you did sounds awful, you still have a life and are young enough to live it. The DH of my uni friend died last year at just 51, which really made us stop and think about our own lives.