Running? Swimming? Or what about buying a treadmill? If you dont like the idea of joining a gym you can buy a cheap treadmill like the R 5.2 on Amazon. It only costs a few hundred pounds and will be cheaper than most gym subscriptions.
A lot of people dont like running, but they actually enjoy running on a treadmill. The good thing is to stave off boredom you can challenge yourself like setting a new PB time for a chosen distance. Or running at a faster speed. You can