Hi Frank,
I thought you were looking for activities specifically related to the campaign such as "Send Email" or "Change Data Value". If you just want custom field value on a per lead basis, then all you need to do is call getLead - http://developers.marketo.com/documentation/soap/getlead/
However, it sounds like you are looking for a Smart Campaign that changed the data value of some IDs. In this case, you will use getLeadActivity or getLeadChanges narrowing down the response to the "Change Data Value" activity only. Below is a snippet of the filter in XML for "getLeadActivity":
<activityFilter>
<includeTypes>
<activityType>ChangeDataValue</activityType>
</includeTypes>
</activityFilter>
In addition, you can use batch size or other date filters such as activityCreatedAt to narrow down the response even more. When you page through the result set, each http callout can give you up to 2 minutes so it shouldn't exceed the timeout limit - http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_callouts_timeouts.htm
If you are still having trouble with this, submit a support case to us and we will help you from there.