I play netball for two hours a week.
before that I'd never heard of men playing netball
Lots of mixed and male teams around.
My only memory of netball is aimlessly running around on a court at school with no idea what I was doing, being yelled at, wearing a "bib", never once getting my hands on the ball, always being the last to be chosen for any team games
It doesn't necessarily sound like it's thee sport for you right now.
I've heard of "Back to Netball"
This is an excellent option. I play in a social netball league and there are players of all levels participating. In the top division, we have players who are truly amazing and play in formal affiliated leagues. In the lower divisions, we have people who are still running around like headless chickens. What's very nice is, over the years,t o see people move up leagues as they improve.
Is the game difficult? It seems very complicated from what I've heard, with "the footwork rule" and many others
In terms of rules, it's relatively simple (compared to something like football). However, it's technically pretty interesting. At lower levels, our umpires are immensely helpful with newbies, and will offer advice etc for recurrent play issues (so if you footwork a lot, you'll get some pointer sat half time).
Do players often get injured? I've heard tales of broken bones, sprained ankles, players running straight into each other and knocking each other over
Over the last couple of seasons, we've had two broken ankles and a shattered collarbone. Missing a couple of games every six weeks, with sprains to ankles/fingers/backs is common.
especially when men play as well
I hate playing with men. Unless they really play netball (rather than get dragged along to mixed leagues with their partner), they are a dangerous combination of superior physical strength and no idea about reasonable contact. I stay away from my opposite number and let him mess up the court rules - obstruction, footwork, offside etc
I wouldn't go immediately into mixed until you feel more confident in your skills.
I have been assured that it wouldn't be competitive at all to start with
He he, you'll get a shock. What goes on on court, stays on court though.
What sort of age of people play? I keep imagining myself surrounded by youngsters fresh out of university (I'm in my late 30's), but do older people play?
I'm 40 and can more than hold my own. Our team is pretty old in league terms, but we can smash the youngsters off court. We took particular pleasure in destroying our local city university team last season; I think they thought they could rock up and use us as practice 
And as I've always said, I might be 40 and not quite as spritely, but I don't give a shit about having a black eye on Facebook
Also, younger girls are a bit shy of bodily contact so if you shove your boobs or groin into their back, they get flustered and you put them off their stride 
that running trainers aren't much good - is that right?
Netball shoes are leather (so not very much give) and relatively heavy, which means you can easily push off the side to change direction. They are also a bit higher on the ankle for support. Runners are far too flexible and you get no quick change of directions because all the "breathable fabrics" don't offer pushing resistance for your foot. I had to play in my runners once last season - my sprints down the court were immense but I had to use a turning circle to change direction, rather than stop/turn/start 
Do it. It's fun, it's tough, it can be bruising. But it's fab exercise.
What is your feeling on position? Are you quick? Can you run forever? Tall/short? A good shot? Good balance?