The grandparent didn't say, "I'll just walk", but rather "I'll at least walk." Give yourself permission to only walk. That way, if you later feel like running

The grandparent didn't say, "I'll just walk", but rather "I'll at least walk." Give yourself permission to only walk. That way, if you later feel like running, you can still get that extra benefit as well, without breaking some promise to yourself, and if you just feel like walking the whole time, no problem there, either. Note that if this relationship of explicitly limited commitments with yourself is preeminent, your goal should be "go out exercising each day" rather than "run 5km each day".