Most of the time I only hear about marathon runners from Kenya. But I have a question. Is there really no other sport to bet on in Kenya?

Bookmaking is very common as everywhere in Africa. I recommend you go to the official local bookmaker's website  - https://1xbet.co.ke/ There are answers to many questions, including legal ones. Registration is required so that you can receive payments from bets. By the way, professional sports in Kenya have long gone beyond marathons.