Can you run with waypoints input on the 645

While not waypoint to waypoint per se, you can use plotaroute.com to create a track with course points via the "directions" feature. They don't have to be directions, though; you can give a descriptive label and change the symbol to be anything you want. If you then bypass Garmin Connect (Mobile) to transfer the GPX file directly to the watch (through the file system with a USB cable, or wirelessly to a Connect IQ app like gimporter from its companion phone app), you can see Next Waypoint, Distance to Next, etc., and get notified as you approach. (Garmin Connect or GCM would override your course points with its own turn-by-turn directions.)

Edit: Actually, I think you may need to export a FIT file (not GPX) to preserve the course points.

Dag. Het lijkt erop dat je ook een Nederlander bent. Klopt dat ? Dat maakt het iets eenvoudiger. Bedankt voor je antwoord Het maken van een route en het op mijn horloge zetten doe ik al. Tot daar snap ik je antwoord. Begrijp ik het goed dat ik, wanneer ik een koers maak, daar al bepaalde waypoints moet aangeven ? En kan ik een koers maken met alleen waypoints ? Op bepaalde trailroutes krijg je dus alleen de waypoints en niet de route per sé,

If you are not Dutch please notify me again and I will translate this message

My surname is Dutch, but I'm two generations away from understanding it. :) Google translated for me.

You'll have to create a route on plotaroute.com with both the track and course points ("directions"). If you don't know the path between waypoints in advance, you can use straight lines, but then you'll likely get many "off course" alerts when navigating. I may have been in error when I said to transfer a GPX file. It may only be the FIT file that contains the course points.

I believe that the Garmin watches do not natively support waypoint to waypoint only. For that you would need a dedicated GPS device (or perhaps a phone). You can choose to navigate to a single waypoint, but once there you'd have to manually select the next one, unless you create a course as I describe above.

Don't have a 645 anymore but I used the dwMap app to do this for hiking rather than running but no reason why it wouldn't work for that as well.

Thanks for the mention jkissane_  Here is more info how to create a set of waypoints for dwMap (up to 10) and how to navigate to them using dwMap on the watch:
https://dynamicwatch.zendesk.com/hc/en-us/articles/360007148272-Waypoints-and-Geocaching

dwMap App Store: https://apps.garmin.com/en-US/apps/2750f280-82f4-4f21-a32c-57acc7ce4870 

Note this feature does require the Premium subscription of dwMap.

I happily paid for Premium when I used a Vivoactive HR. Can dwMap automatically advance to the next waypoint, or does each have to be selected individually?

Thank you for your support Roedema!  dwMap does not auto advance to the next waypoint.  What I would suggest trying is creating add the waypoints, then draw a route that uses our "straight line" routing option to join up the waypoints in the desired sequence.  Then in dwMap follow the route, along the route line, rather than using goto-each point.