Small actions do indeed make a big impact - as long as they happen regularly.
For example, if you jog for 3km / 2 miles every day, then you'd have covered a half marathon every week
Or in other words: you'd be running 100 marathons a year!!!
If you are not ready to run for 3km, how about you start walking just a few minutes and slowly build up? You'll be running your first marathon before long