Welcome to Holland!
Maybe you can contact the local tri club:
De Dolfijn Amsterdam (de Dolfijn means The Dolphin)
Contact email: voorzitter@dolfijntriatlon.nl
So maybe it’s possible to join the training sessions.
And taking a Garmin is a really good idea. There are bike paths everywhere. But it easy to get lost.
TIP: make a loop around Ysselmeer. A big lake. It will take you through many city’s and villages: approx. 280K.
http://www.routeyou.com/route/view/48888/fietsroute-rondje-ijsselmeer.nl
Take a lot of different clothing with you if possible. The temp are 25 degrees Celsius right now. But last summer is was even hotter. But hey, it’s Holland, so lot’s of rain is always possible.
Have fun!