They are really simple to use. You just set up a Jotform account for yourself (free), build your form and when you are happy with it publish it and then copy the link to the published form and include this in your email. Don't bother with other stuff like email notifications and putting it in a web page.
For a basic walk or cycle ride just create a jotform where you collect people's name, email and mobile number.
The other thing you need to do is set the maximum number of submissions to 5 so that after 5 people, the form is disabled and won't take any more submissions (bookings). Here are the instructions for that https://www.jotform.com/ help/137-How-to-Limit- Submissions-in-a-Form.
You can also change the message that comes up when it is full to something like "Event full - try again later in case of cancellations".
To see the submissions (bookings) select the form (when in myforms) and click on submissions.