[throwaway59553]

Lift weights. It's that simple, you just have to find the discipline to do it. If you find that going to the gym is too solitary, and it is, try to find a team sport, in a club near you and start practicing there. Or get a running coach, specifically to prepare you to run those marathons.

>I mainly want become fit enough to run 5ks / half marathons by the end of this year

Things take time, and you need to set more attainable and smaller goals, otherwise by the end of the year, if you fail to run half a marathon it will be heavily demoralizing. Don't try to rush it.

It's hard to not spend time seated while we are working, but after work, just try to have a more active lifestyle. During the week try to walk for 30 mins to 1h. And during the weekends go hiking since you seem to enjoy it, it will be good for both your physical and mental health.