Running is, of course, your best (and cheapest) option if everything else is closed or out of hand. If you are a beginner, again, start small. Don’t try to run for one hour on your first day of training. It might seem manageable at the beginning but you won’t be able to feel your legs at the end of the day (and the following). Also, start with a slower pace to better synchronize your breath.
Ideally, 3 times a week will do just fine. Choose the duration of the training according to your level: if you never run before, then maybe start with just 20 mins the first week, 25 the second, 30 the third and so on. Just bear in mind that only after 30 mins of running your body will start burning fat (sugar comes first!).
If you have problems with your knees or ankles, be extra-careful. Buy comforting shoes, try not to run on asphalt and buy sport elastic tapes to support your joints.
Next one: cycling!