I was on my own with young DCs at one point. I re-engeged with pre-baby still single friends for the odd (every 3 months) night out. Having something to look forward to helps on the lonely nights.
I took a bar job at the weekend, more for the company than the money.
I did lots of journaling (before the days of blogging), have you considered poetry, non fiction, short stories?
I did some research into setting up an online at home business, think about your skills/ hobbies and see if they are marketable.
I paid a teenager to baby sit so I could go to a language leisure evening class at a college, there are so many and lots of young people go too, it's not just for pensioners. If you don't have childcare then you can buy teach yourself CD/books or hire them from the library.
I turned the supermarket shop into an evening out with DC by taking them to the cafe (v cheap) for tea after nursery/work then took my time round the aisles. By the time we got back it'd be bedtime so that was one day a week sorted.
I met another single mum and we swapped babysitting for breaks. Netmums [shocked] is good for meeting people irl.
You could experiment with interior design/DIY and do at home projects.
I never did this but you could use the time to cook batch meals to freeze.
When it's light at night don't think you can't take DCs out in the pram/ for a walk/ go to swings at 8pm if it's dry.
Also bowling is an evening activity you can do alone with young DCs in the eve.