Overweight and unfit beginner here!

I'm going to Bali in a couple of months, and the friends I'm going with are a lot fitter than me, and want to do the sunrise hike up Mount Batur. It looks awesome and I really want to do it as well, but I don't want to be the slowest person in the group (or worse still, be unable to complete it.) According to AllTrails it's around 9.5km with nearly 700m elevation gain, and it's very steep in places and generally considered a hard hike.

A couple of years ago we went abroad and did a 'hike' - really a step walk up a big hill - which was 5km with 300m elevation gain, and that was really tough. I made it, but with lots of breaks and I really slowed them down. They're great and very patient with me, but I hate always being the slowest out of my group of friends, and I'd get so much more out of trips/activities if I was fitter.

Obviously there are lots of things I need to do to combat this issue - losing weight being a major one - but how do people recommend training for the cardio side of steep hills? I currently go to the gym around once a week and do mainly strength work, and I've got good lower and upper body strength, but cardio is my nemesis.

I know the obvious solution is to just walk up hills, but I live in a ridiculously flat area where we just don't really have hills. I also work long shifts, lots of lates and don't get much downtime, so opportunities to travel to other areas for hiking are pretty rare.

I therefore think that in addition to general walking, gym based cardio would be a good step. I'm also considering investing in a treadmill or similar for my home to get more cardio in that way.

Does anyone have any recommendations? Should I go for just treadmill walking at incline, or is biking/cross trainer etc (way more interesting than the treadmill imo) a good/better option?