Running is very popular in Britain and despite its large size London is full of parks and green spaces where people go running. Gym memberships are popular too and there are gyms everywhere. As an obviously fit person you will find London is a great city to explore on foot. Buy yourself the 'A to Z Street Atlas of London'
You can catch a train to numerous seaside resorts from London which are not far away at all, including, Brighton, Broadstairs, Bournemouth. Our national rail system is dense and frequent
www.nationalrail.co.uk
www.railcard.co.uk
www.timeout.com/london
There are plenty of runners in London and gymn membership is popular.
Admittedly, it's not easy to make friends in London. However gym membership might help.
You are never too far from the sea in UK. You can travel from London to the South coast by train very easily. Brighton is a favourite with Londoners. Bournemouth is further away but has nicer beaches.
Londoners are always running. You will see them in every London park during the day. Yes they do join Gyms.
Your workplace will be able to give you all the info you need. In your spare time you can visit all the London attractions.
You will have a great time here.