Welcome to the forum, marizeye. Are there any large open spaces within a short bus or train ride from where you live? (WHEN we're allowed to travel outside our area again.)
I say this mindful of the fact that when I used to lead botany meetings on Hampstead Heath some people complained because we walked a circuit of 5 miles!
Obviously your answer is going to depend on what part of London you're in, but I'm thinking as examples Wanstead in the north, Hampstead Heath in the northwest, Wimbledon Common in the southwest, Thames towpath in the west, and so on. But I believe if you're in the Docklands area you're [censored].
As an aside to any forum members who aren't familiar with London, it's so vast that there are lots of parts you can't reach unless you have 1-2 hours to spare, so some or all of my suggestions to the OP may be useless for him. EDIT: her. Apologies, grovel, grovel!