My answer has always been the same.

What worked for me was an abiding love of the outdoors starting with camping trips with the scouts.

Once you start Hill Walking and getting fit in the process, you will retain the love.

Gymnasiums and Pools are OK to a point, but nothing beats an outdoor hobby.

For my money, the best hobby of the lot has to be cycling. It is a win win situation.

Have a good bike and a good pair of walking boots, then find some mates to go with!