I'm not disagreeing Mark, you're completely right in your principle. I love a session on the rowing machine, if I'm training for a particular race I do it on a Tuesday night. I'm not sure it's totally stress free for the lower legs, you can still put some serious force down through your ankles but it is virtually impact free. In the same session I would do adductor and abductor, upper arms, and neck - very light weights but lots of reps, because in long race my arms and neck suffer before my legs. Another key session for me is the cross-trainer itself but hands clasped behind the back because it unconsciously works the core and improves lower leg stability.
Cycling too..I don't cycle but I know loads of people who cycle more than they run, and turn in very good performances in races
Sounds to me like you're doing fine if you are enjoying it. You've covered a good distance at a good pace if you exclude your walk breaks, in fact you could say you've done three runs. So I woud aim for one run at a time, and do it a lot more slowly, but go for a bit longer timewise. So instead of your three twenty minute sessions, slow down and go for half an hour, then end it there. You'll get plenty of benefit from the run, and you won't wear yourself out. after a few of those, go for forty minutes, and build up (if you want to).
Beneficial? Depends what you want...long term fitness? A fast 5k time? Ability to run long distances without stopping? Weight loss? Whatever you want to do you appear to have got off to a great start, keep it up!