Begin with the Runkeeper Registration/Authorization page for general instructions to get started. This page points to the Application Portal you can use to establish your application. You'll obtain client_id and client_secret by registering a personal application in that portal.
Then obtain code by submitting a request to https://runkeeper.com/apps/authorize with the following parameters directly from the Runkeeper instructions:
client_id: The unique identifier that your application received upon registration
code: coderedirect_uri: The page on your site where the Health Graph API should redirect the user after accepting or denying the access request
I found the easiest way to make this request for my purposes was simply to cobble it into a URL and paste directly into the browser location bar: https://runkeeper.com/apps/authorize?response_type=code&client_id=[MY_CLIENT_ID]&redirect_uri=http%3A%2F%2F[MY_URL.COM]
Replace MY_CLIENT_ID and MY_URL.COM with your values. The URL can be localhost or a domain you control; the request will redirect to it with its returned code value. You'll need to re-use the same value in the form below to obtain a user API token.
Now you need a user token; you'll plug this in to any request for data from the Runkeeper API. There are elegant ways to do this, and then there's this way: A quick HTML form that makes the request for the token. Seriously, I tried all kinds of curl permutations and couldn't get them to work, so I did it this way. It worked. Fill in the values, submit, and get a token back. (I did this all right in a TextMate preview window.)
<html>
<body>
<h1>runkeeper API token obtainer</h1>
<p>Insert values for your API key below, and submit. This form will return a token that can be used for Runkeeper API requests.</p>
<form action="https://runkeeper.com/apps/token" method="POST" enctype="application/x-www-form-urlencoded">
grant_type <input type="text" name="grant_type" value="authorization_code"><br>
code <input type="text" name="code" value=""><br>
client_id <input type="text" name="client_id" value=""><br>
client_secret <input type="text" name="client_secret" value=""><br>
redirect_uri <input type="text" name="redirect_uri" value="http://[MY_URL.COM]"><br>
<input type="submit">
</form>
</body>
</html>
Save the returned token to use in your API calls, and you're good to go.