Mountain Biking Advice Needed Thailand?

Hi,
I am new to travelling in Thailand. I have been doing the guesthouse and local transport thing but it's not active enough for me. I am a keen mountain biker from Yorkshire and I am looking to do a few months on a bike around the country and maybe some mountain bike adventure holidays or tours.

My ideal plan is to have a few guided bike rides here in Bangkok or maybe kanchanaburi and get used to the whole riding on Thai roads thing plus a bit of sightseeing while I'm at it. I know there are a few things to see here in Bangkok like temples and I have heard riding on the long tail boats up and down the Chaopayah river is fun. Also haven't been to Kanchanaburi yet and I would love to Visit the Bridge over The River Kwai and Death Railway. Then I would probably want to hire a bike and head off up to Chiang Mai and do some mountain bike downhill trips up there down Doi Pui or Chiang Dao.

Has anybody on here heard of Khao I-to?