Welcome to the community 
I think it is not unusual to go faster during the first couple of levels. Parts of it since the SRS intervals are lower.
In case you want to slow down there are two basic limits you can set yourself:
- daily number of lessons
- number of items in the apprentice section
As an example, I usually do something between 12-20 lessons per day and try to have ~100 items (or less) in apprentice section. different durtles have different numbers though 
In case you haven’t read it already feel free to check out the ulitimate guide to WK.
In case you want to get a feeling how it will be in the higher levels when going full speed maybe check out the following study logs:
@l_l senpai also gave some feedback and advice in their level 60 post. @GolyBidoof senpai will do in a few days, I guess. Feel free to ask them directly in case you have more questions