15 minutes' walk from my flat there is a glorious stretch of river (Thames) with a towpath known as Barge Walk. This goes between Kingston at one end and Hampton Court Palace at the other, probably about 45 mins to walk all the way down, so it's about a two hour round walk which is one I do often, especially in the spring and autumn. You see all sorts on the way down there: mainly runners and cyclists, but also rowers from schools/clubs, paddle boarders, pleasure cruises, dickheads in skiffs, the works. I even once saw some teenagers diving off a pontoon to swim, which is an interesting choice but each to his own. It's been there since the time of Henry VIII so you can imagine him riding his horse down it (far too lazy to walk). A wonderful mix of history, urbanity, and nature that I don't even have to drive to.
Toggle Spoiler
And on the other side of the palace, there's Bushy Park which is great if you like to catch deer in flagrante delicto.
Toggle Spoiler