I can very much see your point of view, polly. I found it quite difficult to adjust when my ds started school 2 1/2 years ago. It was fine for the first few weeks, as I had time to "catch up" on all the jobs I hadn't had time for during the previous few years. Byt then I started to discover that although I could quite easily fill my time with domestic chores and other activities, they were all solitary activities, and although I wasn't bored, I was definitely lonely.
Ithink it is all very well to say "get a job", but ther are very few jobs which fit well with school hours and holidays (except classroom assistants, vacancies for which, around here, are like gold dust). If you are happy to put your child in childcare some of the time (or have to for financial reasons of needing a job, which it sounds like isn't the case for you)then there are some jobs around.
I also think that some people find it naturally easier than others to make friends in the playground etc. Although I get on fine with most of the other mums, there are very few I would think I knew well enough to ask round, or ask to do something with me. I have tried with one or two, have been round for coffee once, and then never invited me back etc. I also find that the majority of other mums either work, or have younger children, so they are busy going out doing activities with them, while their older dcs are at school.
I do various activities, such as go to the gym, am doing a computer course one morning a week, and help out in ds's school. All of these things help to keep me occupied, and in some cases fit, or educated, which is good. However, the gym (and swimming) is quite solitary, and although I have been quite friendly, I have found I don't have much in common with the other people on the computer course. The school helping is the thing which gives me the most social interaction.
But I still feel that from one end of the week to the other, I don't have much social interaction, although I am quite busy.
I do still belong to a band, which I joined pre-ds, which I go to one evening a week, and enjoy.
I think the best advice I can give, is that if you can't find a job that fits in with the hours you want, then do try either some voluntary work or some sort of club or society which holds a real interest for you, and where you think you may meet like-minded people.
I hope this helps.