On a sunny, dry day, Bewl Water Reservoir just off the A21 in Kent is a lovely spot. A 13-mile hike in total, the path is relatively flat (there is a road section) with glorious views, and shared with cyclists and riders. If trout fishing is your bag, you can fly from the bank or boat. Good luck! OS Explorer map 136 High Weald shows walks in the vicinity plus parking.
Some facts: it can hold over 30 million litres of water; the reservoir covers some 800 acres; it's in an Area of Outstanding Natural Beauty.