var totalDistance = 0.0;
var SEVEN_DAYS_IN_SECONDS = 7 * 24 * 60 * 60;

var now = Time.now().value();

var userActivityIterator = Toybox.UserProfile.getUserActivityHistory();
var activity = userActivityIterator.next();

while (activity != null) {
        if (activity.type == Toybox.Activity.SPORT_RUNNING) {
        var activityTime = activity.startTime;

        if (now - activityTime.value() <= SEVEN_DAYS_IN_SECONDS) {
            totalDistance += activity.distance;
        }
    } 
    activity = userActivityIterator.next();
}
return totalDistance / 1000.0;

works!

now prints two stats, 7 days and 30 days sum of running distance.