Jock itch (tinea cruris) is the irritation of a fungus that normally resides on the skin without causing problems. It is therefore unlikely that you “caught” it from a bicycle seat – in the same way that it’s fairly hard to catch things from toilet seats. Especially if you were wearing trousers at the time.
The inflammation of fungus is caused by heat and moisture. People who are overweight, wearing tight shorts or are wet will be worst affected. As your doctor identifies a 15 minutes bicycle ride shouldn’t be causing you to heat up or sweat to the point it would irritate.
I’m guessing your doctor has advised an anti-fungal cream? Liberal use of this, airing of the affected area and drying thoroughly after a shower or swim will all help quick healing. Talc will also keep the area dry.