So that’s for building muscle, but what about if you wanted to lose a few kilos and increase endurance for long distance running? What would be the way to go to optimise your time?

If you want to increase endurance for running I think the general suggestion is to hit the track and do running? Get your mile time down or similar.

In my personal experience I've found strength training better for losing weight than just cardio but any activity will help a bit. You'll really need to adjust your diet in some way for it though, or at least start counting and keep your calories steady as you do more activity. Trying to outburn what you eat takes like an hour of exercise a day otherwise, it's tough.

Eat less to lose the weight. Tirzepatide or something similar makes that a lot easier.

Tabata (the sprint/recover running technique) was developed, I believe, to increase VO2-max. It should help with overall endurance, and you can go on a long run each week. That would probably be efficient.

Longer slower running burns more fat because your body isn’t forced to use as much glycogen as faster paced running.

I guess the answer for optimizing time is to get a home treadmill if removing the commute to a trail/track will make the timing work.

Overnight hiking. It's not boring, and you get 7 hours of hiking in with a backpack per day.