+ vote for RW's and gemgemx's advice. Intermediate goals are great motivation. You can have great fun with 5Ks, which are in no way inferior to a HM, by the way! They're just different.
You're right to choose another goal aside losing weight, as I've read repeatedly that it's better to lose weight as a by-product of achieving another goal than to pursue it in its own right. Treadmills, hmm, they are often criticised, but to be honest, it's far better to stick to a treadmill routine over winter if that's what you prefer, than to keep saying oh, it's too dark or too cold - easily done! Ideally, you'll do mostly road or trail runs, but don't be too harsh on yourself if you need to come inside.
Totally agree about following a structured plan. But also, don't be afraid of modifying it if you need to. There's no shame in run/walking whenever you need to, and you decide when to take a rest day, not your schedule. Think in terms of weeks and months to make really solid gains, but enjoy those moments when you suddenly notice you can go a bit further or a bit faster.. It's a long game, so it's worth enjoying every step of the way if you can, not just the final race.
Good luck and keep us posted!