The Arlington reservoir near Polegate in East Sussex is one of the reservoirs owned by South East Water which supplies Eastbourne and the surrounding area with a constant supply of water. It also acts as a nature reserve for many species from large birds such as herons to micro-organisms. It's popular with local anglers due to it's trout population. However, I think it's just a nice place to go for a walk on a warm summer day. The views across the water from the dam wall path are splendid.
There is a well signposted circular walk around the reservoir and through the wooded area of the nature reserve called the Osprey walk. It should only take about an hour for a person walking at average speed and is a nice easy-going walk, so nothing very strenuous.
East Sussex County Council have a suggested walk starting at Berwick train station with options for a longer walk around the surrounding countryside which you can download as a pdf.
You could easily adapt this to take a quick diversion into Arlington Village which has a pretty church (St Pancras) and a great pub (The Yew Tree).
Selmeston and the surrounding areas are within easy walking distance of Berwick railway station which is on the line between Eastbourne and Lewes. Trains are operated by Southern Railway and usually stop at least one an hour in each direction.