Vienna is one of the greenest cities in Europe. There are many different parks, plethora of green spaces and trees incorporated into the centuries old architecture design plans. If you head towards the outside districts of Vienna you will find yourself amidst the beautifully curated vineyards. However, if you are looking for a green space to exercise be it running, biking, rollerblading, canoeing or rowing, you head to the Donauinsel.
The Donauinsel is a long and narrow island, lying between the Danube river and the New Donabe. The island is about 21km long. There are trails that circle the island that are perfect for a long bike ride or an adventure run. There are no cars, no stop lights, there is nothing to interrupt your activity. The island offers phenomenal infrastructure to enjoy whatever sport / activity (clearly marked bathrooms, etc)
On the east side of the island, there are bars, restaurants and nightclubs. The island is filled with greenery and has many secluded areas to take in the sun. Getting to the island is easy, there are many bridge that go over the island with easy pedestrian and bike access points. You can enter the island from both the west and the east side of the Danube. There is also Ubahn access point via U1 on the Donauinsel station (map).
For runners, there are paved and non paved trails to explore. For cyclists, the roads are relatively pot hole free. The main issue for fast cyclists is the amount of people that share the roadways. Occasionally you find yourself getting stuck behind people. So if you want a fast non stop cycling experience, I recommend coming during early morning hours.
There are beaches and bbq stations through the island. Please be aware, there is also a nudist beach. The FKK sign indicates the location, so do not be surprised if you come across the large sign.